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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 5 — Feb. 28, 2011
  • pp: 4399–4404
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Beat note stabilization of a 10–60 GHz dual-polarization microlaser through optical down conversion

A. Rolland, M. Brunel, G. Loas, L. Frein, M. Vallet, and M. Alouini  »View Author Affiliations


Optics Express, Vol. 19, Issue 5, pp. 4399-4404 (2011)
http://dx.doi.org/10.1364/OE.19.004399


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Abstract

Down-conversion of a high-frequency beat note to an intermediate frequency is realized by a Mach-Zehnder intensity modulator. Optically-carried microwave signals in the 10–60 GHz range are synthesized by using a two-frequency solid-state microchip laser as a voltage-controlled oscillator inside a digital phase-locked loop. We report an in-loop relative frequency stability better than 2.5 × 10−11. The principle is applicable to beat notes in the millimeter-wave range.

© 2011 Optical Society of America

1. Introduction

The aim of this report is to show that cascading an optical intensity modulator after a dual-frequency laser permits to completely lift the constraints due to millimeter-wave electrical devices. In particular, we study a dual-polarization microlaser emitting at 1064 nm with a widely tunable beat note. We intend to show that an opto-electronic PLL (OEPLL) including an integrated intensity modulator yields a fully tunable phase-locked beat signal over the whole 10–60 GHz range with a high spectral purity. The principle of the two-frequency down-conversion is exposed in Section 2. The experimental OEPLL is described in Section 3, and the results on the stabilization of the beat note are presented in Section 4. Conclusions and perspectives are given in Section 5.

2. Optical down conversion

Fig. 1 Optical down-conversion. (a) Set-up: dual-frequency microchip laser; P, polarizer; MZM, intensity modulator. (b) MZM transmission when Vbias = Vπ/2, leading to the output spectrum (c). (d) MZM transmission when Vbias =0 leading to the output spectrum (e).

Here, in order to benefit from the whole beat note tunability, we propose to use an intensity modulator before the photodiode, as explained in Fig. 1. The linearly polarized dual-frequency beam is injected into the MZM that can be exploited in two different configurations. On the one hand, if the MZM is biased in quadrature to yield a maximum first-harmonic modulation at fRF [see Fig. 1(b) where Vbias = Vπ/2], then the output beam contains the frequency spectrum depicted in Fig. 1(c). In this case, one can see that the IF, obtained between the two central, closely spaced, sidebands, is a low-frequency image of the beat note. On the other hand, if the MZM is biased in phase in order to maximize the second-harmonic response at 2fRF [see Fig. 1(d) where Vbias = 0], then the same IF is obtained with a twice as high beat note [see Figs. 1(e)]. In this case again, the IF provides a low-frequency image of the beat note. Under both biasing conditions, the MZM acts on the two optical frequencies and creates a down-converted IF at frequency fi = ΔνpfRF, with p = 2 (quadrature) or p = 4 (phase). It is important to note that here, contrary to microwave-photonic schemes using a MZM as a frequency generator [2

2. J. Yao, “Microwave photonics,” J. Lightwave Technol. 27(3), 314–335 (2009). [CrossRef]

], the sidebands produce a low-frequency error signal, not the microwave signal. A low-pass photodiode can then be used to exploit this down-converted IF signal in a PLL.

In order to investigate the IF optical power impinging on a photodiode through this method, we start from the well-known analysis of the MZM [14

14. G. Qi, J. Yao, J. Seregelyi, S. Paquet, and C. Bélisle, “Generation and distribution of a wide-band continuously tunable millimeter-wave signal with an optical external modulation technique,” IEEE Trans. Microw. Theory Tech. 53(10), 3090–3097 (2005). [CrossRef]

]. We write Iin the optical intensity of one mode at the entrance of the modulator, ωRF the electrical angular frequency driving the modulator, and a and b the bias and RF voltages, respectively, normalized to Vπ. The intensity Iout at the output of the modulator can then be expressed by a sum of Bessel-functions. By biasing in quadrature, i.e., a = 0.5, the even-order functions vanish and the output intensity becomes
Iout(t)=Iin2Iin[J1(bπ)sin(ωRFt)+J3(bπ)sin(3ωRFt)+J5(bπ)sin(5ωRFt)+],
(1)
while by biasing in phase, i.e. a = 0, the odd-order functions vanish leading to
Iout(t)=Iin2+Iin2J0(bπ)+Iin[J2(bπ)cos(2ωRFt)+J4(bπ)cos(4ωRFt)+].
(2)
Considering now equal intensities for the two modes simultaneously at the modulator input, one can calculate an output beat power at IF of IIF(t)=IinJ12(bπ)cos(2πfit) with fi = Δν – 2fRF in the first case. The maximal amplitude is obtained when = 1.8, leading to J12(bπ)=0.33. In the second case, one finds IIF(t)=IinJ22(bπ)cos(2πfit) with fi = Δν – 4fRF. The maximal amplitude is then obtained when = 3.1, leading to J22(bπ)=0.23. This simple analysis shows that a MZM can provide an optical solution to the problem of down-converting a millimeter-wave beat signal. It is worthwhile mentioning that the IF can be maintained in a low-frequency region, say below 1 GHz, whatever Δν, simply by changing the RF frequency on the modulator. We now describe the OEPLL that relies on this principle.

3. Optoelectronic phase-locked loop

The complete loop is schematized in Fig. 2. The free-space laser output beam is sent through a polarizing optical isolator OI and injected into a polarization-maintaining (PM) fiber coupler C. The input polarizer of OI is adjusted at 45° with respect to x and y. About 1 mW of optical power is injected in the loop. In order to be able to adjust the IF beat power level, the dual-frequency beam is amplified by a PM ytterbium-doped fiber amplifier YDFA. Amplifier pumping is adjusted to get an incident optical power impinging on the modulator equal to 10 dBm. Note that, if one could use an optimized microlaser delivering 100 mW of optical power, 20% of the power could be extracted for stabilization purpose without requesting optical amplification. Our MZM is a 10 GHz-bandwidth integrated LiNbO3 from Photline Technologies. The modulator output is detected by a pigtailed 45 GHz bandwidth photodiode D. Before describing the remainder of the loop, let us describe the experimental spectrum illustrating the down-conversion method. Figure 3 shows the electrical spectrum of the resulting photocurrent when the laser beat is chosen at 21 GHz (method described in Section 4) and the MZM is operated in quadrature at fRF = 8.5 GHz. It appears clearly that, in addition to the beat note at Δν, one finds the IF of interest at fi = Δν – 2fRF, here 4 GHz, as expected. The extra peaks at fRF and 2fRF correspond to extra beats between sidebands and carriers. These, as well as the main beat at Δν, are easily filtered out, leaving the IF for generating a clean error signal.

Fig. 2 Schematic of the experiemental setup. See text for details
Fig. 3 Illustration of the optical down-conversion. Electrical spectrum of the photocurrent with Δν = 21 GHz and fRF = 8.5 GHz, yielding fi = 4 GHz.

Following Ref. [8

8. M. Vallet, M. Brunel, and M. Oger, “RF photonic synthesizer,” Electron. Lett. 43(25), 1437–1438 (2007). [CrossRef]

], this IF signal is fed into a common digital phase-locked frequency synthesizer (model LMX2430 from National Semiconductor). It mainly consists in a frequency/phase detector, with an input bandwidth equal to 0.25–2.5 GHz, followed by a charge pump and a loop filter. Both the digital synthesizer and the MZM driver are synchronized to the same reference source, here a quartz oscillator at fr = 10 MHz. The error signal is applied to the LiTaO3 electro-optic crystal inside the laser through a high-voltage amplifier (HVA). Finally, we use a PIC microcontroller in order to program the values N and R of the input and reference dividers, respectively. It sets the command frequency to fi=NR×fr. The aim is hence to obtain a programmable beat note at frequency Δν=pfRF+NR×fr, with an integer p = 2 (quadrature) or p = 4 (phase).

4. Stabilization results

When the loop is left open, the free running laser beat note has a typical 100 kHz linewidth on a millisecond measurement time scale but fluctuates at a rate of typically a few hundred MHz over a few minutes [9

9. A. Rolland, L. Frein, M. Vallet, M. Brunel, F. Bondu, and T. Merlet, “40 GHz photonic synthesizer using a dual-polarization microlaser,” IEEE Photon. Technol. Lett. 22(23), 1738–1740 (2010). [CrossRef]

]. These fluctuations, originating from pump noise and environmental disturbances, are illustrated in Fig. 4(a). We emphasize the fact that such a short microchip laser offers a strong thermal sensitivity due to the thermo-optic effect in the LiTaO3 [5

5. M. Brunel, A. Amon, and M. Vallet, “Dual-polarization microchip laser at 1.53 μm,” Opt. Lett. 30(18), 2418–2420 (2005). [CrossRef] [PubMed]

, 9

9. A. Rolland, L. Frein, M. Vallet, M. Brunel, F. Bondu, and T. Merlet, “40 GHz photonic synthesizer using a dual-polarization microlaser,” IEEE Photon. Technol. Lett. 22(23), 1738–1740 (2010). [CrossRef]

]. It makes the laser easily tunable over the whole beat note tuning range, namely 10–60 GHz. Figure 4(b) depicts the experimental measures of the beat frequency versus temperature. A high gain of 3 GHz/K is measured. Once the beat frequency is thermally chosen, the loop permits to stabilize it electrically, as is now finally shown.

Fig. 4 (a) Free-running beat fluctuation over 10 minutes, measured with the MaxHold function of the electrical spectrum analyzer. (b) Thermo-optic tuning of the beat note. (c)–(d) Stabilized IF signals when the servo-loop is closed. Resolution bandwidth 1 Hz, video averaging 10. In (c), Δν = 20 GHz, fRF = 9.75 GHz, a = 0.5 (quadrature) ). In (d), Δν = 40 GHz, fRF = 9.875 GHz, a = 1 (phase).

The dual-frequency laser acts as the voltage-controlled oscillator (VCO) in the PLL. Due to the electro-optic effect of the LiTaO3, the measured VCO gain is about 1 MHz/V. Taking into account the 300 V maximum output voltage of the HVA, the loop has a lock-in range of typically 300 MHz. Besides, the bandwidth of the loop filter is 100 kHz and the channel spacing of the digital synthesizer is 1 MHz (R = 10). In order to show the efficiency of the method in both quadrature and phase biasing configurations, we first set a free-running beat at 20 GHz, a MZM frequency fRF = 9.75 GHz, and a biasing voltage of a = 0.5 (quadrature). When the loop is closed, the beat is then stabilized at any frequency chosen by the operator through the PIC. For example, when N = 500 (Δν = 20GHz), Fig. 4(c) shows the IF signal on a 160 Hz span. The measured instrument-limited 1 Hz linewidth illustrates the quality of this stabilization method. Furthermore, by simply changing the laser temperature by a few degrees, we can obtain a free-running beat at 40 GHz. Then, to phase-lock at 40 GHz, a simple change of the MZM biasing voltage at a = 0 (phase) gives us again an IF in the synthesizer bandwidth, while all other parameters are left unchanged. Again, the beat note is stabilized with the same efficiency, as proved by Fig. 4(d). Any other beat frequency between 10 and 40 GHz can be locked using this method, by steps of 1 MHz (the channel spacing). The actual limit in our set-up is the 10 GHz bandwidth of the MZM, which prevents us from carrying on the demonstration up to 60 GHz. Besides, the sweeping rate of the synthesis is measured to be 1.25 MHz/μs (corresponding to a 50 MHz frequency step in a lock-in time of 40 μs).

5. Conclusion

In conclusion, we have demonstrated a new principle of stabilization loop of any beat note between two optical frequencies in the range of tens of GHz. It includes a MZM which efficiently down-converts in the optical domain the beat note to an IF in the sub-GHz range. A digital PLL allows then to stabilize the IF whose fluctuations are a replica of the beat note fluctuations. Such an opto-electronic phase locked loop (OEPLL) is implemented on a dual frequency microchip laser providing a beat note tunable from 10 GHz up to 60 GHz. The resulting relative stability is measured to be better than 1 Hz over 40 GHz, i.e., 2.5 × 10−11. Furthermore, we show that biasing the MZM in phase advantageously enables to double the operation range of the OEPLL. This is illustrated at 20 GHz and 40 GHz using a 10 GHz cut-off frequency MZM. Besides, the OEPLL operation principle inherently offers a large frequency tunability with a single servo-loop by adjusting the MZM modulation frequency.

Acknowledgments

The authors thank G. Danion for preliminary work on the microlaser. This work was funded in part by Région Bretagne (contract PONANT and an ARED grant) and by the Délégation Générale pour l’Armement (contract ARAMOS).

References and links

1.

J. Capmany and D. Novak, “Microwave photonics combines two worlds,” Nat. Photonics 1, 319–330 (2007). [CrossRef]

2.

J. Yao, “Microwave photonics,” J. Lightwave Technol. 27(3), 314–335 (2009). [CrossRef]

3.

M. Alouini, B. Benazet, M. Vallet, M. Brunel, P. Di Bin, F. Bretenaker, A. Le Floch, and P. Thony, “Offset phase locking of Er:Yb:glass laser eigenstates for RF photonics applications,” IEEE Photon. Technol. Lett. 13(4), 367–369 (2001). [CrossRef]

4.

G. Pillet, L. Morvan, M. Brunel, F. Bretenaker, D. Dolfi, M. Vallet, J.-P. Huignard, and A. Le Floch, “Dual-frequency laser at 1.5 μm for optical distribution and generation of high-purity microwave signals,” J. Lightwave Technol. 26(15), 2764–2773 (2008). [CrossRef]

5.

M. Brunel, A. Amon, and M. Vallet, “Dual-polarization microchip laser at 1.53 μm,” Opt. Lett. 30(18), 2418–2420 (2005). [CrossRef] [PubMed]

6.

A. McKay and J. M. Dawes, “Tunable terahertz signals using a helicoidally polarized ceramic microchip laser,” IEEE Photon. Technol. Lett. 21(7), 480–482 (2009). [CrossRef]

7.

R. Wang and Y. Li, “Dual-polarization spatial-hole-burning-free microchip laser,” IEEE Photon. Technol. Lett. 21(17), 1214–1216 (2009). [CrossRef]

8.

M. Vallet, M. Brunel, and M. Oger, “RF photonic synthesizer,” Electron. Lett. 43(25), 1437–1438 (2007). [CrossRef]

9.

A. Rolland, L. Frein, M. Vallet, M. Brunel, F. Bondu, and T. Merlet, “40 GHz photonic synthesizer using a dual-polarization microlaser,” IEEE Photon. Technol. Lett. 22(23), 1738–1740 (2010). [CrossRef]

10.

K. J. Williams, L. Goldberg, R. D. Esman, M. Dagenais, and J. F. Weller, “6–34 GHz offset phase-locking of Nd:YAG 1319 nm nonplanar ring lasers,” Electron. Lett. 25(18), 1242–1243 (1989). [CrossRef]

11.

Y. Li, J. C. Vieira, S. M. Goldwasser, and P. R. Herczfeld, “Rapidly tunable millimeter-wave optical transmitter for Lidar-Radar,” IEEE Trans. Microw. Theory Tech. 49(10), 2048–2054 (2001). [CrossRef]

12.

H. R. Rideout, J. S. Seregelyi, S. Paquet, and J. Yao, “Discriminator-aided optical phase-lock loop incorporating a frequency down-conversion module,” IEEE Photon. Technol. Lett. 18(22), 2344–2346 (2006). [CrossRef]

13.

S. Hisatake, Y. Nakase, K. Shibuya, and T. Kobayashi, “Generation of flat power-envelope terahertz-wide modulation sidebands from a continuous-wave laser based on an external electro-optic phase modulator,” Opt. Lett. 30(7), 777–779 (2005). [CrossRef] [PubMed]

14.

G. Qi, J. Yao, J. Seregelyi, S. Paquet, and C. Bélisle, “Generation and distribution of a wide-band continuously tunable millimeter-wave signal with an optical external modulation technique,” IEEE Trans. Microw. Theory Tech. 53(10), 3090–3097 (2005). [CrossRef]

OCIS Codes
(140.3580) Lasers and laser optics : Lasers, solid-state
(060.5625) Fiber optics and optical communications : Radio frequency photonics

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: January 14, 2011
Revised Manuscript: February 14, 2011
Manuscript Accepted: February 14, 2011
Published: February 22, 2011

Citation
A. Rolland, M. Brunel, G. Loas, L. Frein, M. Vallet, and M. Alouini, "Beat note stabilization of a 10–60 GHz dual-polarization microlaser through optical down conversion," Opt. Express 19, 4399-4404 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-5-4399


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References

  1. J. Capmany, and D. Novak, "Microwave photonics combines two worlds," Nat. Photonics 1, 319-330 (2007). [CrossRef]
  2. J. Yao, "Microwave photonics," J. Lightwave Technol. 27(3), 314-335 (2009). [CrossRef]
  3. M. Alouini, B. Benazet, M. Vallet, M. Brunel, P. Di Bin, F. Bretenaker, A. Le Floch, and P. Thony, "Offset phase locking of Er:Yb:glass laser eigenstates for RF photonics applications," IEEE Photon. Technol. Lett. 13(4), 367-369 (2001). [CrossRef]
  4. G. Pillet, L. Morvan, M. Brunel, F. Bretenaker, D. Dolfi, M. Vallet, J.-P. Huignard, and A. Le Floch, "Dual-frequency laser at 1.5 μm for optical distribution and generation of high-purity microwave signals," J. Lightwave Technol. 26(15), 2764-2773 (2008). [CrossRef]
  5. M. Brunel, A. Amon, and M. Vallet, "Dual-polarization microchip laser at 1.53 μm," Opt. Lett. 30(18), 2418-2420 (2005). [CrossRef] [PubMed]
  6. A. McKay, and J. M. Dawes, "Tunable terahertz signals using a helicoidally polarized ceramic microchip laser," IEEE Photon. Technol. Lett. 21(7), 480-482 (2009). [CrossRef]
  7. R. Wang, and Y. Li, "Dual-polarization spatial-hole-burning-free microchip laser," IEEE Photon. Technol. Lett. 21(17), 1214-1216 (2009). [CrossRef]
  8. M. Vallet, M. Brunel, and M. Oger, "RF photonic synthesizer," Electron. Lett. 43(25), 1437-1438 (2007). [CrossRef]
  9. A. Rolland, L. Frein, M. Vallet, M. Brunel, F. Bondu, and T. Merlet, "40 GHz photonic synthesizer using a dual-polarization microlaser," IEEE Photon. Technol. Lett. 22(23), 1738-1740 (2010). [CrossRef]
  10. K. J. Williams, L. Goldberg, R. D. Esman, M. Dagenais, and J. F. Weller, "6-34 GHz offset phase-locking of Nd:YAG 1319 nm nonplanar ring lasers," Electron. Lett. 25(18), 1242-1243 (1989). [CrossRef]
  11. Y. Li, J. C. Vieira, S. M. Goldwasser, and P. R. Herczfeld, "Rapidly tunable millimeter-wave optical transmitter for Lidar-Radar," IEEE Trans. Microw. Theory Tech. 49(10), 2048-2054 (2001). [CrossRef]
  12. H. R. Rideout, J. S. Seregelyi, S. Paquet, and J. Yao, "Discriminator-aided optical phase-lock loop incorporating a frequency down-conversion module," IEEE Photon. Technol. Lett. 18(22), 2344-2346 (2006). [CrossRef]
  13. S. Hisatake, Y. Nakase, K. Shibuya, and T. Kobayashi, "Generation of flat power-envelope terahertz-wide modulation sidebands from a continuous-wave laser based on an external electro-optic phase modulator," Opt. Lett. 30(7), 777-779 (2005). [CrossRef] [PubMed]
  14. G. Qi, J. Yao, J. Seregelyi, S. Paquet, and C. Bélisle, "Generation and distribution of a wide-band continuously tunable millimeter-wave signal with an optical external modulation technique," IEEE Trans. Microw. Theory Tech. 53(10), 3090-3097 (2005). [CrossRef]

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