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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 4 — Feb. 19, 2007
  • pp: 1612–1620

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Wavelength locking of CW and Q-switched Er3+ microchip lasers to acetylene absorption lines using pump-power modulation

Marc Brunel and Marc Vallet  »View Author Affiliations


Optics Express, Vol. 15, Issue 4, pp. 1612-1620 (2007)
http://dx.doi.org/10.1364/OE.15.001612


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Abstract

We show that modulating the diode-pump power of a microchip solid-state laser enables to lock its wavelength to a reference molecular line. The method is applied to two different types of Er,Yb:glass monolithic microchip lasers operating at 1.53 μm. First, wavelength locking of a continuous-wave dual-polarization microchip laser to acetylene absorption lines is demonstrated, without using any additional modulator, internal or external. We then show that, remarkably, this simple method is also suitable for stabilizing a passively Q-switched microchip laser. A pulsed wavelength stability of 10-8 over 1 hour is readily observed. Applications to lidars and to microwave photonics are discussed.

© 2007 Optical Society of America

1. Introduction

Wavelength stabilization of solid-state lasers is mandatory in areas as diverse as spectroscopy [1–3

W. Demtröder, Laser spectroscopy, 3d ed . (Springer, Berlin, 2003).

], wavelength-division multiplexed telecommunications [4

P. Laporta, S. Taccheo, S. Longhi, C. Svelto, and P. De Natale, “Frequency locking of tunable Er:Yb microlasers to absorption lines of 13C2H2 in the 1540-1550 nm wavelength interval,” Appl. Phys. Lett. 71,2731–2733 (1997). [CrossRef]

], or remote sensing [5

G. J. Koch, M. Petros, J. Yu, and U. N. Singh, “Precise frequency control of a single-frequency pulsed Ho:Tm:YLF laser,” Appl. Opt. 41,1718–1721 2002). [CrossRef] [PubMed]

,6

K. Ertel, H. Linné, and J. Bösenberg, “Injection-seeded pulsed Ti:sapphire laser with novel stabilization scheme and capability of dual-wavelength operation,” Appl. Opt. 44,5120–5126 (2005). [CrossRef] [PubMed]

]. For continuous-wave (cw) lasers, wavelength stabilization to molecular lines is usually performed by comparing the actual laser wavelength with a chosen reference wavelength. The resulting error signal is then fed back to an actuator such as a piezo-electric element that corrects the cavity length [1–4

W. Demtröder, Laser spectroscopy, 3d ed . (Springer, Berlin, 2003).

]. Besides, for stabilizing a pulsed laser, its cavity length is also piezo-electrically controlled in order to have one mode coinciding with a cw master laser wavelength. The latter is itself locked to a reference line [5

G. J. Koch, M. Petros, J. Yu, and U. N. Singh, “Precise frequency control of a single-frequency pulsed Ho:Tm:YLF laser,” Appl. Opt. 41,1718–1721 2002). [CrossRef] [PubMed]

,6

K. Ertel, H. Linné, and J. Bösenberg, “Injection-seeded pulsed Ti:sapphire laser with novel stabilization scheme and capability of dual-wavelength operation,” Appl. Opt. 44,5120–5126 (2005). [CrossRef] [PubMed]

]. In all these lasers, cw or Q-switched,optical pumping of the solid-state active medium induces thermo-optic effects and dilatation that are usually undesirable. But in the case of monolithic lasers, such as microchip or non-planar ring lasers [7

J. J. Zayhowski, “Microchip lasers,” Opt. Mater. 11,255–267 (1999). [CrossRef]

,8

R. L. Byer, “Diode laser-pumped solid-state lasers,” Science 239,742–747 (1988). [CrossRef] [PubMed]

], pump-induced thermal effects can become useful because the short cavity length is significantly modified by the pump power, as demonstrated in [9–11

J. A. Keszenheimer, E. J. Balboni, and J. J. Zayhowski, “Phase-locking of 1.32 μm microchip lasers through the use of pump-diode modulation,” Opt. Lett. 17,649–651 (1992). [CrossRef] [PubMed]

]. To the aim of stabilizing the wavelength of a microchip laser to a molecular line, one can hence wonder whether the pump power could be used as the modulator and/or the actuator in a servo-locking loop, in order to keep the inherent simplicity of the laser structure. Furthermore, one may also ask whether such an actuator permits to lock the wavelength of a Q-switched microchip laser against a molecular line without adding any other component.

The purpose of this article is therefore to show the wavelength servo-locking of cw or pulsed microchip solid-state lasers to molecular reference lines, by using the pump power as the only wavelength controller, without adding any external element. We study in particular the locking of erbium lasers to acetylene absorption lines at 1.53 μm. The paper is organized as follows. In section 2, we investigate experimentally and theoretically a two-frequency cw microchip laser. Then, in section 3, wavelength locking of a passively Q-switched single-frequency laser is demonstrated. We summarize and discuss some applications in section 4.

2. Dual-polarization microchip laser

While single-frequency monolithic lasers are now customary, novel two-frequency lasers are promising for microwave photonics applications [12

M. Brunel, A. Amon, and M. Vallet, “Dual-polarization microchip laser at 1.53 μm,” Opt. Lett. 30,2418–2420 (2005). [CrossRef] [PubMed]

,13

L. Morvan, M. Alouini, J. Bourderionnet, J. Le Gouët, D. Dolfi, and J. P. Huignard, “Widely tunable twofrequency Nd:YAG laser,” in CLEO/QELS and PhAST, Technical Digest (Optical Society of America, 2005), paper CF01.

]. This is why we consider here,without loss of generality, an Er,Yb:glass microchip laser which contains a birefringent crystal. It operates in a dual-polarization regime at two tunable wavelengths around 1.53 μm,and delivers a beat note from 0 to 60 GHz [12

M. Brunel, A. Amon, and M. Vallet, “Dual-polarization microchip laser at 1.53 μm,” Opt. Lett. 30,2418–2420 (2005). [CrossRef] [PubMed]

]. After briefly recalling the laser characteristics,we focus on wavelength variations with respect to pump power modulation and detail the servo-loop needed to lock one of the two emitted wavelengths to a molecular line.Experimental results on (i) wavelength locking and, (ii) the resulting beat note stabilization,are discussed.

2.1 Description of the laser

The microchip laser is pumped by a pigtailed laser diode at 975 nm, as shown in Fig. 1. A couple of lenses L1 images the fiber output into the microchip laser, leading to a pump beam radius equal to rp = 23 μm inside the active medium. The latter consists of a slice of Er,Yb:glass (QX from Kigre, Inc.), with erbium and ytterbium doping concentration of 8×1019 cm-3 and 2×1021 cm-3, respectively. Its thickness La is equal to 0.19 mm. It is glued to a slice of x-cut LiTaO3 crystal (thickness Lb = 0.13 mm), as sketched in the inset of Fig. 2(a). This birefringent element yields two orthogonally polarized eigenstates aligned with the ordinary and extraordinary axes of the crystal, labeled (o) and (e) in the following. The length of the laser has been calculated to ensure monomode operation on both eigenstates. The input face is high-reflection coated and the output face transmits 1% at 1.53 μm. The temperature T of the laser was set at 18°C using the thermo-electric cooler holding the microchip heat sink.

Fig. 1. Experimental set-up. LD, pump laser diode; L1,2, lenses; OI, optical isolator; D, photodiode; ϵ,error signal; Ω, modulation frequency. Inset: experimental absorption spectrum of 13C2H2 recorded from 1532.5 to 1537.5 nm by passing light from an amplified-spontaneous emission source at 1.53 μm through the cell. P-lines of interest here are labeled from 1 to 6.

When the incident pump power is set at 200 mW, the laser emits a 5 mW TEM00 beam with an estimated mode radius rm=50 μm located inside the microchip. As shown in Fig. 2(a), the optical spectrum consists in two wavelengths λ0 and λe, with balanced powers, corresponding to the two orthogonal eigenpolarizations. Such a two-frequency regime leads, through a polarizer aligned at 45° with respect to the neutral axes of the crystal, to a beat note ∆V equal to the difference of the eigenfrequencies Ve-Vo [12

M. Brunel, A. Amon, and M. Vallet, “Dual-polarization microchip laser at 1.53 μm,” Opt. Lett. 30,2418–2420 (2005). [CrossRef] [PubMed]

]. For example, at T=18°C, ∆V is around 40 GHz [see Fig. 2(b)].

In free-running operation, each wavelength experiences variations that are measured to be of a few pm (hundreds of MHz) on short term scale, i.e., over one minute. Besides, long term,i.e., one hour, variations are of the order of tens of pm (a few GHz) because of additional thermal and mechanical drifts. The aim of the following is to stabilize one of the emitted wavelengths, say λe, to an acetylene absorption line.

Fig. 2. (a) Dual-polarization optical spectrum. Optical spectrum analyzer resolution bandwidth is 0.1 nm. Inset: schematics of the composite microchip. (b) Electrical power spectrum of the beat note at 40 GHz obtained by sending the output beam through a polarizer on a 45 GHz-bandwidth photodiode (resolution bandwidth is 100 kHz).

2.2 Wavelength tuning by pump-power modulation

Let us first recall that changing the temperature of a solid-state laser modifies the optical length L of the laser, hence its operation frequency. In the case of our two-frequency laser,there are two optical paths Lo,e = naLa + no,eLb associated with the two eigenpolarizations.The frequency change dVo,e with respect to the temperature T (averaged over the volume of the laser mode) writes dVo,e = -Λo,edT, with

Λ o,e= V o,e L o,e [ na La ( 1 na ∂na ∂T+ αa)+ n o,e Lb ( 1 n o,e ∂n o,e ∂T+ α o,e)]
(1)

na (resp.no,e) and αa(resp. αo,e) are the index of refraction and the expansion coefficient of the active medium (resp. birefringent element). At λ=1.53μm, QX glass parameters are na=1.5, αa = 8.2×10−6 K−1 and ∂na/∂T = −1.0×10−6 K−1, while LiTaO3 parameters are no,e ≃ 2.1, αo = 21×10−6 K−1, αe = 5.7×10−6 K−1, ∂no/∂T = 1.6×10−6 K−1, and ∂ne/∂T = 24×10−6 K−1. These numerical values lead to thermo-optic frequency tuning rates equal to Λo= −2.4 GHz/K (corresponding to a wavelength tuning rate ∆λo/∆T of 19 pm/K) and Λe= −2.9 GHz/K (23 pm/K).

Due to the excess thermal energy deposited by the pump beam inside the active medium,the laser temperature can be controlled by the pump power. In the case of Nd:YAG microchip lasers, an expression for the temperature response to pump-power modulation was theoretically derived [9

J. A. Keszenheimer, E. J. Balboni, and J. J. Zayhowski, “Phase-locking of 1.32 μm microchip lasers through the use of pump-diode modulation,” Opt. Lett. 17,649–651 (1992). [CrossRef] [PubMed]

]. Here we extend this model to dual-polarization lasers by assuming that the local temperature of the birefringent element adiabatically follows the temperature variations of the active medium and that the absorption is uniformly distributed along the whole cavity path. For a pump-power modulation frequency Ω higher than a few Hz, the temperature response of the microchip is equal to

∣ dTdP∣= κ 4π ( ka La+ kb Lb) Ci2 ( Ω Ωo)+ ( π2−Si ( Ω Ωo))2.
(2)

The steady-state limit (Ω→0) writes

lim Ω→0∣ dTdP∣≅ κ 4π ( ka La+ kb Lb) [ 0.58+Ln ( 2rb2 ( rm2+ rp2))].
(3)

In Eqs. (2) and (3), ka and kb are the thermal conductivities of the active medium and of the birefringent crystal, respectively, rb is the distance from the cavity axis to the heat sink, Ci and Si are the cosine and sine integrals [14

M. Abramowitz and I. E. Stegun, Handbook of mathematical functions , (Dover, New York, 1965).

], Ω0 is a cut-off frequency proportional to the thermal diffusivity of the material [9

J. A. Keszenheimer, E. J. Balboni, and J. J. Zayhowski, “Phase-locking of 1.32 μm microchip lasers through the use of pump-diode modulation,” Opt. Lett. 17,649–651 (1992). [CrossRef] [PubMed]

], and κ corresponds to the heat-generating efficiency of the pump. For QX glass and LiTaO3, ka and kb are equal to 0.85 and 4.6 W.m-1.K-1, respectively. rb is equal to 500 μm. Finally, the magnitude of the frequency response to pump power is derived by combining Eqs. (1) and (2), giving

∣ dv o,edP∣ (Ω)= − Λ o,e ∣ dTdP∣ (Ω).
(4)

Eq.(4) will be used to fit the experimental results in the following.

Using an optical spectrum analyzer, we have first measured the steady-state wavelength tuning rate with respect to the pump-power. The experimental slope is equal to 5.8 pm/mW (-0.74 GHz/mW). Then, in order to record the magnitude of the laser frequency response to pump-power modulation, an AC signal at Ω was sent to the current driver of the pump diode,leading to a peak-to-peak pump power modulation of 9 mW. Figure 3 shows the corresponding experimental values of |dVe/dP| as a function of Ω. Using Ω0 = 750 Hz and κ = 0.2, Eq.(4) yields a good fit to the experimental points. We point out that the discrepancy at low frequencies is due to the fact that Eq. (2) no longer applies [9

J. A. Keszenheimer, E. J. Balboni, and J. J. Zayhowski, “Phase-locking of 1.32 μm microchip lasers through the use of pump-diode modulation,” Opt. Lett. 17,649–651 (1992). [CrossRef] [PubMed]

]. Note also that, using these parameter values in Eq. (3), we find a theoretical steady-state frequency response to the pump power equal to -0.9 GHz/mW, in agreement with the experimental value.

Fig. 3. Frequency response to pump-power modulation. Dots: experimental points; full line: theoretical curve using Eq.(4) with the parameters given in text.

2.3 Locking to an acetylene absorption line

The laser output is collimated by a lens L2 (see Fig.1). It then splits to an output beam and to the beam used for wavelength-locking. By rotating the optical isolator OI, we select one eigenpolarization, for example the extraordinary one. The transmitted single-wavelength beam then goes through a 5 cm long sealed gas cell. It is filled with pure 13C2H2 at a pressure of 50 Torr (6.7 kPa). The cell presents absorption lines associated with the P-branch of the (V1+V3) band [15

A. A. Madej, J. E. Bernard, A. J. Alcock, A. Czajkowski, and S. Chepurov, “Accurate absolute frequencies of the V1+V3 band of 13C2H2 determined using an infrared mode-locked Cr:YAG laser frequency comb,” J. Opt. Soc. Am. B 23,741–749 (2006). [CrossRef]

], as shown in the inset of Fig. 1. Then, the laser beam is sent to an InGaAs photodiode. In order to reduce the spurious intensity noise due to undamped relaxation oscillations at frequencies around 100 kHz [12

M. Brunel, A. Amon, and M. Vallet, “Dual-polarization microchip laser at 1.53 μm,” Opt. Lett. 30,2418–2420 (2005). [CrossRef] [PubMed]

], the photodiode output is filtered with a lowpass 3dB point at 130 Hz. Finally this filtered signal is sent to a lock-in circuit in order to lock λe to one absorption line.

The mean spacing between P lines of 13C2H2 being equal to 70 GHz [15

A. A. Madej, J. E. Bernard, A. J. Alcock, A. Czajkowski, and S. Chepurov, “Accurate absolute frequencies of the V1+V3 band of 13C2H2 determined using an infrared mode-locked Cr:YAG laser frequency comb,” J. Opt. Soc. Am. B 23,741–749 (2006). [CrossRef]

], we adjust the temperature T over ±20°C to tune λe around an absorption line, for instance the P(5) line at a wavelength λabs=1535.98 nm. Figure 4(a) shows the 30% absorption dip of the P(5) line with respect to the pump power. This demonstrates that λe can be finely tuned through an absorption line using small pump power variations. The observed 1.8 mW full width at half maximum (FWHM) yields a width of 1.3 GHz for the P(5) line, in agreement with pressurebroadened acetylene linewidths found in the literature [4

P. Laporta, S. Taccheo, S. Longhi, C. Svelto, and P. De Natale, “Frequency locking of tunable Er:Yb microlasers to absorption lines of 13C2H2 in the 1540-1550 nm wavelength interval,” Appl. Phys. Lett. 71,2731–2733 (1997). [CrossRef]

].

To servo-lock the wavelength of the laser, λe is tuned to the center of the absorption line and then dithered by modulating the power of the pump beam at Ω =7 Hz. We choose this modulation frequency in order to keep a high gain of 0.4 GHz/mW (see Fig. 3). The modulation amplitude is set at 0.25 mW, yielding peak-to-peak frequency modulation of 200 MHz. A home-made lock-in amplifier then provides the error signal ϵ which is fed back to the current driver of the laser diode, as depicted in Fig. 1. The overall typical DC gain of the electronics circuit, from the photodiode input to the laser-diode output, is 15 mW/mW and the lock-in response time is equal to 3.3 s. Figure 4(b) reports the open loop error signal ϵ when λe is tuned through the absorption line.

Then, closing the feedback loop efficiently cancels all fluctuations of the extraordinary laser frequency, as shown in Fig. 4(c). One can see that λe is locked for hours to the acetylene absorption line. Moreover, the frequency standard deviation is estimated from this recording of ϵ(t) to be 14 MHz (0.1 pm) over 20 hours, leading to a frequency stability better than 10−7. We also checked that the same frequency accuracy was obtained when λe was tuned to other absorption lines, namely P(2), P(3), and P(4), or when the ordinary wavelength λo was used to servo-control the dual-polarization laser.

Fig. 4. (a) Transmission of the cell while slowly tuning the extraordinary wavelength around the P(5) line. (b) Corresponding open-loop error signal (averaged over 10 scans). (c) Long-term frequency fluctuations, as derived from the closed-loop error signal.

2.4 Beat note stability

In our case of a two-frequency laser, the stability of the beat note is an important issue for microwave photonics applications. In free-running operation, the observed line-width is smaller than the spectrum analyzer resolution, here 100 kHz [see Fig. 2(b)], due to the intrinsic low phase noise of diode-pumped solid-state lasers [9

J. A. Keszenheimer, E. J. Balboni, and J. J. Zayhowski, “Phase-locking of 1.32 μm microchip lasers through the use of pump-diode modulation,” Opt. Lett. 17,649–651 (1992). [CrossRef] [PubMed]

]. But the beat note presents a large drift induced by thermal and mechanical drifts. It can be estimated by holding the maximum of the beat note peak, as depicted in Fig. 5. We measure a free-running drift of the order of 1 GHz over two hours. Then, when the loop is closed, the variation range drops below 100 MHz over 2 hours, as shown in Fig. 5(b). Indeed, servo-locking one wavelength stabilizes the other one, because they share the same cavity. The residual frequency variations of 100 MHz appear due to the optical wavelength dithering at 7 Hz because the thermo-optic tuning rates Λo and Λe are slightly different. Besides, the beat-note line-width as well as the laser intensity noise is unaffected when the loop is closed. Further reduction of the jitter is now needed in order to fully characterize the intensity and phase noises of the microwave signal.

These results prove the possibility of microchip wavelength stabilization to a molecular line by using pump-power modulation. It is tempting to apply this simple technique to the case of Q-switched pulsed lasers, for which wavelength stabilization is usually tricky [5

G. J. Koch, M. Petros, J. Yu, and U. N. Singh, “Precise frequency control of a single-frequency pulsed Ho:Tm:YLF laser,” Appl. Opt. 41,1718–1721 2002). [CrossRef] [PubMed]

,6

K. Ertel, H. Linné, and J. Bösenberg, “Injection-seeded pulsed Ti:sapphire laser with novel stabilization scheme and capability of dual-wavelength operation,” Appl. Opt. 44,5120–5126 (2005). [CrossRef] [PubMed]

].This is the aim of the next section.

Fig. 5. Electrical power spectrum of the beat note, recorded over 2 hours holding the peak maximum.(a) free-running. (b) servo-loop closed.

3. Stabilization of a passively Q-switched microchip laser

Using the same experimental set-up (see Fig.1), we investigate a passively Q-switched microlaser. We describe first the components of this laser and its output beam characteristics and, second, the modifications of the feedback loop that must be made in order to take the pulsed feature of this laser into account. Experimental results and conclusion on the performances of the servo-locking then follow.

3.1 Description of the laser

The active medium is a 0.5-mm long glass plate from the same composition as in section 2.One of its faces is used as the cavity input mirror: R < 5% at 975 nm and R > 99.9% at 1.53 μm. Its output face is antireflection coated at 1.53 μm. In order to Q-switch the laser, we use a strontium lanthanum aluminate (ASL) plate [16

V. Lupei, G. Aka, and D. Vivien, “Highly efficient 0.84 slope efficiency, 901 nm, quasi-two-level laser emission of Nd in strontium lanthanum aluminate,” Opt. Lett. 31,1064–1066 (2006). [CrossRef] [PubMed]

], doped with a 1.5% at. concentration of Co2+, which provides saturable absorption at 1.5 μm [17

N. D. Lai, M. Brunel, F. Bretenaker, B. Ferrand, and L. Fulbert, “Two-frequency Er-Yb:glass microchip laser passively Q-switched by a Co:ASL saturable absorber,” Opt. Lett. 28,328–330 (2003). [CrossRef] [PubMed]

]. We choose a c-cut sample of thickness 160 μm to yield a small-signal intensity transmission of 99 % at 1.53 μm. The cavity is closed by a plane mirror transmitting 0.5% at 1.53 μm. All the elements of the cavity are closely stacked together, as depicted in the inset of Fig. 6(a). It results in a compact microchip cavity whose total optical length is 1 mm. As in section 2, the cavity is made stable by thermal lensing in the active medium.

Under these operating conditions, the laser threshold is reached at an incident pump power of 320 mW. The average output power of the laser increases linearly with the absorbed pump power, reaching a value of 5 mW at the maximum pump power of 490 mW (slope efficiency of 3 %). The laser oscillates in a single-longitudinal-mode at all pump powers because of the étalon effect inside the 160 μm thick absorber. At a pump power of 420 mW, the laser wavelength is equal to 1534.8 nm, as shown by the experimental optical spectrum in Fig. 6(a).In these conditions, the laser output consists in a train of pulses as depicted in Fig. 6(b). The pulse period is 390 μs, giving a repetition rate f = 2.6 kHz. Due to the short cavity length, the pulse duration is as short as 6 ns [see inset of Fig. 6(b)]. The average output power is measured to be 4 mW. From this, we deduce that the pulse peak power is about 270 W and the pulse energy is 1.6 μJ.

3.2 Q-switched laser stabilization

Using the temperature-tunability of this laser, we first choose the thermoelectric cooler temperature for the emitted wavelength to reach an absorption line of isotopic acetylene, P(3) at 1534.89 nm for example. We then verify that the wavelength is linearly tunable with the pump power and we measure an experimental slope of 3.5 pm/mW (-0.45 GHz/mW). In order to take advantage of this pump-power to wavelength conversion in the pulsed regime, the stabilization loop has to be slightly modified. First, we use a capacitive, large area Ge photodiode (D in Fig. 1). Second, in order to avoid saturation of D, the laser power is decreased by inserting neutral density filters on the optical path, leading to an intensity transmission of about 1.4 %. Third, to compensate for this optical attenuation, the photodiode preamplifier gain is enhanced by 15 dB. Finally, the preamplifier low-pass cut-off frequency is set at 0.3 kHz. Hence the input of the lock-in amplifier is fed with a smooth signal proportional to the average power transmitted by the acetylene cell.

Fig. 6. (a) Q-switched laser optical spectrum with a 0.1 nm resolution bandwidth. It is longitudinally monomode. Inset: microchip laser structure. (b) Typical pulse train. Inset: temporal zoom on one pulse.

The laser wavelength is tuned around the P(3) line using the pump power. The resulting open loop error signal is reproduced in Fig. 7(a), where the useful width appears to be about 3 mW around the line center. Then, when the loop is closed, the laser wavelength gets locked to the absorption maximum of this 13C2H2 line, as displayed in Fig. 7(b). This recording evidences the efficiency of the method to correct for the drifts. This is, to our knowledge, the first stabilization of a Q-switched microchip laser to a reference line.

Fig. 7. (a) Open-loop error signal obtained when the laser wavelength is pump-power tuned around the P(3) line. (b) Frequency difference between laser frequency V and the targeted 13C2H2 absorption line frequency Vabs, as deduced from ϵ. The loop is closed at 21 min.

We have checked that the pulse shape and energy remain constant. The pump power dithering only induces residual 7 Hz periodic variations of the repetition rate of the order of 5%. The 60-minute closed-loop recording of Fig. 7(b) gives a frequency standard deviation of only 2 MHz, i.e., a stability of 10-8. We verified that the laser frequency can be locked to other acetylene resonance lines, namely P(2), P(4), and P(5). Finally, it is worth noting that this simple technique leads to a long-term frequency standard deviation value which is less than the estimated 30 MHz bandwidth of such 6 ns pulses emitted by our Q-switched laser.

4. Conclusion

The wavelength locking of cw and Q-switched erbium-doped microchip lasers to acetylene absorption lines has been demonstrated using pump-power modulation, without adding any actuator or modulator, internal or external. Because of the short cavity which enhances the influence of the thermo-optic effect, we find a high power-to-frequency conversion efficiency.A simple feedback to the current supply of the pump diode then permits to lock the wavelength of either a dual-polarization cw laser or a single-frequency passively Q-switched laser.

Such a technique is straightforward to implement for a given microchip laser. In the Q-switched regime, the method could be suitable for lidars. In order to obtain more output power than in this work, one could use amplified microchip lasers [18

F. Imkenberg, J. Barenz, H. D. Tholl, A. Malinowski, K. Furusawa, and D. J. Richardson, “Microchip laser master-oscillator Er/Yb-doped fiber-power-amplifier emitting 158 μJ pulses with a duration of 4.5 ns,” Proc. CLEO-Europe 2003, 317 (2003), paper CL5-6.

] or high-power mini-lasers [19

A. Agnesi, F. Pirzio, G. Reali, and G. Piccinno, “Subnanosecond diode-pumped passively Q-switched Nd:GdVO4 laser with peak power > 1 MW,” Appl. Phys. Lett. 89,101120 (2006). [CrossRef]

]. Application to a differential absorption lidar (DIAL) using a dual-wavelength pulsed microchip laser is now under study. Finally, the same principle can be extended to Doppler-free saturation spectroscopy using recently developed gas-filled photonic crystal fibers [20

F. Benabid, F. Couny, J. C. Knight, T. A. Birks, and P. St. J. Russell, “Compact, stable, and efficient allfibre gas cells using hollow-core photonic crystal fibres,” Nature 434,488–491 (2005). [CrossRef] [PubMed]

,21

J. Henningsen, J. Hald, and J. C. Petersen, “Saturated absorption in acetylene and hydrogen cyanide in hollow-core photonic bandgap fibers,” Opt. Express 13,10475–10482 (2005). [CrossRef] [PubMed]

].

Acknowledgments

The authors wish to thank L. Frein and C. Hamel for technical assistance, and L. Fulbert, N. Benjemaa, and M. Himdi for their kind help. This work was performed in the framework of the Contrat de Plan Etat-Région Bretagne.

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5.

G. J. Koch, M. Petros, J. Yu, and U. N. Singh, “Precise frequency control of a single-frequency pulsed Ho:Tm:YLF laser,” Appl. Opt. 41,1718–1721 2002). [CrossRef] [PubMed]

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K. Ertel, H. Linné, and J. Bösenberg, “Injection-seeded pulsed Ti:sapphire laser with novel stabilization scheme and capability of dual-wavelength operation,” Appl. Opt. 44,5120–5126 (2005). [CrossRef] [PubMed]

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R. L. Byer, “Diode laser-pumped solid-state lasers,” Science 239,742–747 (1988). [CrossRef] [PubMed]

9.

J. A. Keszenheimer, E. J. Balboni, and J. J. Zayhowski, “Phase-locking of 1.32 μm microchip lasers through the use of pump-diode modulation,” Opt. Lett. 17,649–651 (1992). [CrossRef] [PubMed]

10.

P. Thony and E. Molva, “1.55 μm-wavelength cw microchip lasers,” OSA TOPS on Advanced Solid-State Lasers Vol. 1, S. A. Payne and C. Pollock, Eds., (Optical Society of America, Washington DC, 1996), pp.296–300.

11.

M. Heurs, V. M. Quetschke, B. Willke, K. Danzmann, and I. Freitag, “Simultaneously suppressing frequency and intensity noise in a Nd:YAG nonplanar ring oscillator by means of the current-lock technique,” Opt. Lett. 29,2148–2150 (2004). [CrossRef] [PubMed]

12.

M. Brunel, A. Amon, and M. Vallet, “Dual-polarization microchip laser at 1.53 μm,” Opt. Lett. 30,2418–2420 (2005). [CrossRef] [PubMed]

13.

L. Morvan, M. Alouini, J. Bourderionnet, J. Le Gouët, D. Dolfi, and J. P. Huignard, “Widely tunable twofrequency Nd:YAG laser,” in CLEO/QELS and PhAST, Technical Digest (Optical Society of America, 2005), paper CF01.

14.

M. Abramowitz and I. E. Stegun, Handbook of mathematical functions , (Dover, New York, 1965).

15.

A. A. Madej, J. E. Bernard, A. J. Alcock, A. Czajkowski, and S. Chepurov, “Accurate absolute frequencies of the V1+V3 band of 13C2H2 determined using an infrared mode-locked Cr:YAG laser frequency comb,” J. Opt. Soc. Am. B 23,741–749 (2006). [CrossRef]

16.

V. Lupei, G. Aka, and D. Vivien, “Highly efficient 0.84 slope efficiency, 901 nm, quasi-two-level laser emission of Nd in strontium lanthanum aluminate,” Opt. Lett. 31,1064–1066 (2006). [CrossRef] [PubMed]

17.

N. D. Lai, M. Brunel, F. Bretenaker, B. Ferrand, and L. Fulbert, “Two-frequency Er-Yb:glass microchip laser passively Q-switched by a Co:ASL saturable absorber,” Opt. Lett. 28,328–330 (2003). [CrossRef] [PubMed]

18.

F. Imkenberg, J. Barenz, H. D. Tholl, A. Malinowski, K. Furusawa, and D. J. Richardson, “Microchip laser master-oscillator Er/Yb-doped fiber-power-amplifier emitting 158 μJ pulses with a duration of 4.5 ns,” Proc. CLEO-Europe 2003, 317 (2003), paper CL5-6.

19.

A. Agnesi, F. Pirzio, G. Reali, and G. Piccinno, “Subnanosecond diode-pumped passively Q-switched Nd:GdVO4 laser with peak power > 1 MW,” Appl. Phys. Lett. 89,101120 (2006). [CrossRef]

20.

F. Benabid, F. Couny, J. C. Knight, T. A. Birks, and P. St. J. Russell, “Compact, stable, and efficient allfibre gas cells using hollow-core photonic crystal fibres,” Nature 434,488–491 (2005). [CrossRef] [PubMed]

21.

J. Henningsen, J. Hald, and J. C. Petersen, “Saturated absorption in acetylene and hydrogen cyanide in hollow-core photonic bandgap fibers,” Opt. Express 13,10475–10482 (2005). [CrossRef] [PubMed]

OCIS Codes
(140.3500) Lasers and laser optics : Lasers, erbium
(140.3540) Lasers and laser optics : Lasers, Q-switched
(140.3580) Lasers and laser optics : Lasers, solid-state

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: October 10, 2006
Revised Manuscript: December 21, 2006
Manuscript Accepted: December 21, 2006
Published: February 19, 2007

Citation
Marc Brunel and Marc Vallet, "Wavelength locking of CW and Q-switched Er3+ microchip lasers to acetylene absorption lines using pump-power modulation," Opt. Express 15, 1612-1620 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-4-1612


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References

  1. W. Demtröder, Laser spectroscopy, 3d ed. (Springer, Berlin, 2003).
  2. A. Arie, S. Schiller, E. K. Gustafson, and R. L. Byer, "Absolute frequency stabilization of diode-laser-pumped Nd:YAG lasers to hyperfine transitions in molecular iodine," Opt. Lett. 17, 1204-1206 (1992). [CrossRef] [PubMed]
  3. P. Laporta, S. Taccheo, M. Marano, O. Svelto, E. Bava, G. Galzerano, and C. Svelto, "Amplitude and frequency stabilized solid-state lasers in the near infrared," J. Phys. D: Appl. Phys. 34, 2396-2407 (2001). [CrossRef]
  4. P. Laporta, S. Taccheo, S. Longhi, C. Svelto, and P. De Natale, "Frequency locking of tunable Er:Yb microlasers to absorption lines of 13C2H2 in the 1540-1550 nm wavelength interval," Appl. Phys. Lett. 71, 2731-2733 (1997). [CrossRef]
  5. G. J. Koch, M. Petros, J. Yu, and U. N. Singh, "Precise frequency control of a single-frequency pulsed Ho:Tm:YLF laser," Appl. Opt. 41, 1718-1721 (2002). [CrossRef] [PubMed]
  6. K. Ertel, H. Linné, and J. Bösenberg, "Injection-seeded pulsed Ti:sapphire laser with novel stabilization scheme and capability of dual-wavelength operation," Appl. Opt. 44, 5120-5126 (2005). [CrossRef] [PubMed]
  7. J. J. Zayhowski, "Microchip lasers," Opt. Mater. 11, 255-267 (1999). [CrossRef]
  8. R. L. Byer, "Diode laser-pumped solid-state lasers," Science 239, 742-747 (1988). [CrossRef] [PubMed]
  9. J. A. Keszenheimer, E. J. Balboni, and J. J. Zayhowski, "Phase-locking of 1.32 μm microchip lasers through the use of pump-diode modulation," Opt. Lett. 17, 649-651 (1992). [CrossRef] [PubMed]
  10. P. Thony and E. Molva, "1.55 μm-wavelength cw microchip lasers," OSA TOPS on Advanced Solid-State Lasers Vol. 1, S. A. Payne and C. Pollock, Eds., (Optical Society of America, Washington DC, 1996), pp. 296-300.
  11. M. Heurs, V. M. Quetschke, B. Willke, K. Danzmann, and I. Freitag, "Simultaneously suppressing frequency and intensity noise in a Nd:YAG nonplanar ring oscillator by means of the current-lock technique," Opt. Lett. 29, 2148-2150 (2004). [CrossRef] [PubMed]
  12. M. Brunel, A. Amon, and M. Vallet, "Dual-polarization microchip laser at 1.53 μm," Opt. Lett. 30, 2418-2420 (2005). [CrossRef] [PubMed]
  13. L. Morvan, M. Alouini, J. Bourderionnet, J. Le Gouët, D. Dolfi, and J. P. Huignard, "Widely tunable two-frequency Nd:YAG laser," in CLEO/QELS and PhAST, Technical Digest (Optical Society of America, 2005), paper CF01.
  14. M. Abramowitz and I. E. Stegun, Handbook of mathematical functions, (Dover, New York, 1965).
  15. A. A. Madej, J. E. Bernard, A. J. Alcock, A. Czajkowski, and S. Chepurov, "Accurate absolute frequencies of the v1+v3 band of 13C2H2 determined using an infrared mode-locked Cr:YAG laser frequency comb," J. Opt. Soc. Am. B 23, 741-749 (2006). [CrossRef]
  16. V. Lupei, G. Aka, and D. Vivien, "Highly efficient 0.84 slope efficiency, 901 nm, quasi-two-level laser emission of Nd in strontium lanthanum aluminate," Opt. Lett. 31, 1064-1066 (2006). [CrossRef] [PubMed]
  17. N. D. Lai, M. Brunel, F. Bretenaker, B. Ferrand, and L. Fulbert, "Two-frequency Er-Yb:glass microchip laser passively Q-switched by a Co:ASL saturable absorber," Opt. Lett. 28, 328-330 (2003). [CrossRef] [PubMed]
  18. F. Imkenberg, J. Barenz, H. D. Tholl, A. Malinowski, K. Furusawa, and D. J. Richardson, "Microchip laser master-oscillator Er/Yb-doped fiber-power-amplifier emitting 158 μJ pulses with a duration of 4.5 ns," Proc. CLEO-Europe 2003, 317 (2003), paper CL5-6.
  19. A. Agnesi, F. Pirzio, G. Reali, and G. Piccinno, "Subnanosecond diode-pumped passively Q-switched Nd:GdVO4 laser with peak power > 1 MW," Appl. Phys. Lett. 89, 101120 (2006). [CrossRef]
  20. F. Benabid, F. Couny, J. C. Knight, T. A. Birks, and P. St. J. Russell, "Compact, stable, and efficient all-fibre gas cells using hollow-core photonic crystal fibres," Nature 434, 488-491 (2005). [CrossRef] [PubMed]
  21. J. Henningsen, J. Hald, and J. C. Petersen, "Saturated absorption in acetylene and hydrogen cyanide in hollow-core photonic bandgap fibers," Opt. Express 13, 10475-10482 (2005). [CrossRef] [PubMed]

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