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  • Vol. 36, Iss. 7 — Apr. 1, 2011
  • pp: 1197–1199
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Narrowband Er:YAG nonplanar ring oscillator at 1645 nm

Da-Wun Chen, Paul M. Belden, Todd S. Rose, and Steven M. Beck  »View Author Affiliations


Optics Letters, Vol. 36, Issue 7, pp. 1197-1199 (2011)
http://dx.doi.org/10.1364/OL.36.001197


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Abstract

We report 1645 nm narrowband operation of a monolithic Er:YAG nonplanar ring oscillator resonantly pumped at 1532 nm . Unidirectional cw power up to 0.5 W was obtained with a measured linewidth of 21 kHz .

© 2011 Optical Society of America

In recent years, Er:YAG lasers have been developed for eye-safe emission at 1645nm for both cw and Q-switched applications, including remote sensing, range finding, spectroscopy, lidar, and wavelength conversion [1

1. Y. Young, S. Setzler, K. Snell, P. Budni, T. Pollak, and E. Chicklis, Opt. Lett. 29, 1075 (2004). [CrossRef] [PubMed]

, 2

2. K. Spariosu, V. Leyva, R. Reeder, and M. Klotz, IEEE J. Quantum Electron. 42, 182 (2006). [CrossRef]

, 3

3. D. Shen, J. Sahu, and W. Clarkson, Opt. Lett. 31, 754 (2006). [CrossRef] [PubMed]

]. Certain applications, such as coherent lidar, require narrowband operation at modest to high power levels, which necessitates seeding with a stable, single-frequency source. The nonplanar ring oscillator (NPRO) has these qualities and has been demonstrated to operate at wavelengths near 1μm (Nd:YAG) [4

4. T. J. Kane and R. L. Byer, Opt. Lett. 10, 65 (1985). [CrossRef] [PubMed]

, 5

5. E. J. Zang, J. P. Cao, Y. Li, T. Yang, and D. M. Hong, Opt. Lett. 32, 250 (2007). [CrossRef]

], 2.01μm (Tm:YAG) [6

6. C. Gao, M. Gao, Y. Zhang, Z. Lin, and L. Zhu, Opt. Lett. 34, 3029 (2009). [CrossRef] [PubMed]

], and 2.09μm (Ho:YAG) [7

7. B.-Q. Yao, X.-M. Duan, D. Fang, Y.-J. Zhang, L. Ke, Y.-L. Ju, Y.-Z. Wang, and G.-J. Zhao, Opt. Lett. 33, 2161 (2008). [CrossRef] [PubMed]

]. In this Letter, we report, to our knowledge, the first demonstration of an Er:YAG NPRO at 1645nm. The device is pumped with an Er fiber laser, as opposed to laser diodes, which enables optimum matching of the pump and resonator modes. This reduces spatial hole-burning and yields a more stable single-mode device without using an undoped section of material.

The design of our Er:YAG NPRO follows that described in [8

8. E. J. Zang, J. P. Cao, M. Zhong, C. Li, N. Shen, D. Hong, L. Cui, Z. Zhu, and A. Liu, Appl. Opt. 41, 7012 (2002). [CrossRef] [PubMed]

], which relaxes the alignment tolerance yet still enables sufficient differential loss to be achieved between counter propagating waves in the presence of a magnetic field (Verdet effect) for unidirectional operation. A schematic of the crystal with the dimensions 14mm(width)×3mm(height)×13mm(length) is shown in Fig. 1. The crystal material is 2  at. % doping Er:YAG. As depicted in the figure, the optical path ABCD forms a nonplanar ring with an approximately 30mm round trip.

The pump laser beam is incident at point A with a 45° angle of incidence. The ring laser output also exits at point A at an angle of 45° as shown. The relative tilt angles of the surfaces containing points A, B, C, and D are such that a 45° angle is achieved between the two optical planes defined by points A, B, and D and B, C, and D. Using 45° achieves a good balance between relaxing the alignment sensitivity and maximizing the differential loss, which minimizes the magnetic field required for unidirectional operation. The facets containing B, C, and D are uncoated and polished flat, allowing for total internal reflection, while the surface containing A, designated the input/output coupler (IC/OC), is flat for one crystal and 5cm concave (internally) for a second. The IC/OC of both crystals was coated for high transmission of the 1532nm p-polarized pump and a reflectivity (R) of 99% for the s-polarized 1645nm laser output. The high R at 1645nm was chosen to ensure a low threshold and narrow linewidth, rather than to optimize power and efficiency. Additionally, the IC/OC coating has a higher loss (R<90%) for p-polarized light at 1645nm to ensure a linearly polarized output.

The NPRO crystals were held in a copper mount that was thermoelectrically cooled to 15°C during testing. In order to achieve the single directional oscillation, a permanent magnetic field, H, was applied to the NPRO in the direction shown in Fig. 1. NdFeB magnets (grade N42) with a permanent field of 0.4T were stacked as needed. (The Verdet constant for the YAG crystal at 1645nm is calculated to be 6.9×105rad/mm-T [9

9. E. Munin, J. A. Roversi, and A. Balbin Villaverde, J. Phys. D 25, 1635 (1992). [CrossRef]

], which is about 2.6 times smaller than at 1μm. ) As depicted in Fig. 2, the laser crystal was pumped with an IPG cw linearly polarized Er fiber laser at 1532.4nm, with a spectral linewidth of 0.2nm (FWHM). The pump output was oriented to be p-polarized and mode-matched by a lens L1 into the NPRO crystal at 45°. An optical isolator (OI) was used to prevent laser feedback into the pump. Laser output at 1645nm was collimated by a lens L2 and separated from the reflected pump with a dichroic mirror (DM). An Ando AQ6319 optical spectral analyzer (OSA) with a 0.01nm (1.1GHz) resolution was used to confirm single longitudinal mode operation but was inadequate for determining the actual linewidth. Quantitative measurements were performed using a delayed self-heterodyne method [10

10. H. Tsuchida, Opt. Lett. 15, 640 (1990). [CrossRef] [PubMed]

], as described below.

Overall, the performance of the NPRO with the flat OC was slightly better than that with the curved OC, so only results for that former crystal will be presented. It was observed that a single 0.4T magnet was sufficient to yield a stable unidirectional single frequency up to 0.1W, whereas four magnets were required for unidirectional operation at 0.5W. We further observed that stable single-frequency operation also necessitated that the pump mode diameter be smaller than 170μm. Optimum performance was achieved when the pump diameter was set to 120μm, yielding an optical-to-optical efficiency of 11% at 0.45W of output. The performance of the crystal with this pumping spot is shown in Fig. 3. Figure 4 shows a comparison of the NPRO’s spectra for single-mode (blue solid curve) and multimode (red dashed curve) operation.

The 0.05nm mode separation measured with the OSA agreed with the predicted value for the 30mm round trip ring cavity. Fine tuning of the Er:YAG NPRO was achieved by changing the crystal temperature. When the crystal temperature was changed from 15°C to 13°C, the laser wavelength was continuously tuned without mode hopping from 1645.258 to 1645.214nm (5GHz), as shown in Fig. 5. The wavelength tuning rate is 0.022nm/°C (2.5GHz/°C). The NPRO output power increased linearly from 189mW at 15°C to 198mW at 13°C.

The NPRO output beam quality was measured as a function of power with a Shack–Hartmann wavefront sensor manufactured by FLIR Systems. Figure 6 shows that beam quality M2 was less than 1.2 over the entire range. An image of the NPRO far-field beam profile is shown as an inset of the Fig. 6.

As mentioned above, we used a delayed self- heterodyne method to measure NPRO output linewidth. The setup is shown in Fig. 7. The laser output was split by a 90/10 beam splitter (BS). The reflected beam was coupled into a 25km fiber for a time delay τd of about 125μs, which is sufficiently longer than the expected laser coherence time. The transmitted beam was frequency shifted by an acousto-optic modulator (AOM) operating at 27.13MHz. The two beams were recombined by a fiber coupler. The beat signal was detected by an amplified InGaAs photodetector and displayed on an RF spec trum analyzer. The resolution of the delayed self- heterodyne method can be estimated from the expression [0.4log(ΔfBτd)+0.6]/τd, where ΔfB is the spectral analyzer bandwidth [9

9. E. Munin, J. A. Roversi, and A. Balbin Villaverde, J. Phys. D 25, 1635 (1992). [CrossRef]

]. For our setup, τd=125μs and ΔfB=0.5kHz, which yields a resolution of 8.6kHz.

An RF spectrum of the self-heterodyne signal is shown in Fig. 8 at the NPRO output power 100mW over a 10s sweep time. This spectrum shows a central peak and relaxation oscillation sidebands at ±146kHz. The amplitudes of both sidebands are 25dB below that of the central feature. Relaxation oscillations are common in most solid-state lasers, occurring when the upper laser level lifetime is much longer than the cavity decay time. The central peak of 3dB bandwidth in Fig. 8 was measured to be 30kHz, which corresponds to a deconvolved FWHM linewidth of 21kHz, assuming a Gaussian lineshape. We also observed that the laser linewidth and relaxation oscillation frequency increased to 26kHz and 173kHz, respectively, at a higher laser power level of 150mW. At present we are unable to perform the measurements on a shorter time scale, which could reveal an even narrower instantaneous linewidth.

In summary, we have demonstrated for the first time, to our knowledge, a 0.5W single-frequency Er:YAG NPRO operating at 1645nm that is resonantly pumped by a Er fiber laser at 1532.4nm. The device was tunable over a 5GHz range without mode hopping and had a bandwidth of 21kHz (over a 10s interval). The wavelength tuning rate of 0.022nm/°C (2.5GHz/°C) is obtained from the experiment. The beam quality of the device was observed to be near-diffraction-limited. As such, this device is a viable source for injection seeding, which is needed to obtain high-power narrowband eye-safe emission for various lidar and other remote sensing applications.

This work was supported under the Aerospace Corporation’s Mission Oriented Investigation and Experimentation program, funded by the U.S. Air Force Space and Missile Systems Center under Contract No. FA8802-04-C-0001.

Fig. 1 Schematic of Er:YAG NPRO crystal.
Fig. 2 Experimental layout for the NPRO pumped by an Er fiber laser.
Fig. 3 NPRO single-frequency power performance data comparison: pump beam diameter 120μm (triangles with blue solid curve) versus 140μm (squares with red dotted curve). Smaller pump beam diameter yields more efficient output and stable single-frequency operation.
Fig. 4 NPRO output mode structures measured by Ando OSA: red dotted curve (multimode) versus blue solid curve (single mode).
Fig. 5 NPRO single-mode cw tuning as a function of the crystal temperature. The tuning rate of 0.022nm/°C is measured.
Fig. 6 NPRO output beam quality M2 data versus output power. Inset: image of output far-field beam profile.
Fig. 7 Experimental setup for NPRO delayed self-heterodyne linewidth measurement.
Fig. 8 Delayed self-heterodyne signal recorded by an RF spectral analyzer at 100mW. NPRO single-frequency operation. Full width at 3dB was measured to be 30kHz.
1.

Y. Young, S. Setzler, K. Snell, P. Budni, T. Pollak, and E. Chicklis, Opt. Lett. 29, 1075 (2004). [CrossRef] [PubMed]

2.

K. Spariosu, V. Leyva, R. Reeder, and M. Klotz, IEEE J. Quantum Electron. 42, 182 (2006). [CrossRef]

3.

D. Shen, J. Sahu, and W. Clarkson, Opt. Lett. 31, 754 (2006). [CrossRef] [PubMed]

4.

T. J. Kane and R. L. Byer, Opt. Lett. 10, 65 (1985). [CrossRef] [PubMed]

5.

E. J. Zang, J. P. Cao, Y. Li, T. Yang, and D. M. Hong, Opt. Lett. 32, 250 (2007). [CrossRef]

6.

C. Gao, M. Gao, Y. Zhang, Z. Lin, and L. Zhu, Opt. Lett. 34, 3029 (2009). [CrossRef] [PubMed]

7.

B.-Q. Yao, X.-M. Duan, D. Fang, Y.-J. Zhang, L. Ke, Y.-L. Ju, Y.-Z. Wang, and G.-J. Zhao, Opt. Lett. 33, 2161 (2008). [CrossRef] [PubMed]

8.

E. J. Zang, J. P. Cao, M. Zhong, C. Li, N. Shen, D. Hong, L. Cui, Z. Zhu, and A. Liu, Appl. Opt. 41, 7012 (2002). [CrossRef] [PubMed]

9.

E. Munin, J. A. Roversi, and A. Balbin Villaverde, J. Phys. D 25, 1635 (1992). [CrossRef]

10.

H. Tsuchida, Opt. Lett. 15, 640 (1990). [CrossRef] [PubMed]

OCIS Codes
(140.3070) Lasers and laser optics : Infrared and far-infrared lasers
(140.3460) Lasers and laser optics : Lasers
(140.3500) Lasers and laser optics : Lasers, erbium
(140.3560) Lasers and laser optics : Lasers, ring
(140.3570) Lasers and laser optics : Lasers, single-mode
(140.3580) Lasers and laser optics : Lasers, solid-state

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: January 5, 2011
Revised Manuscript: February 14, 2011
Manuscript Accepted: February 20, 2011
Published: March 28, 2011

Citation
Da-Wun Chen, Paul M. Belden, Todd S. Rose, and Steven M. Beck, "Narrowband Er:YAG nonplanar ring oscillator at 1645 nm," Opt. Lett. 36, 1197-1199 (2011)
http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-36-7-1197


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