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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 Photonics Technology Department, Electronics and Photonics Laboratory, The Aerospace Corporation, 2350 East El Segundo Boulevard, El Segundo, California 90245, USA *Corresponding author:
[email protected] Received January 5, 2011; revised February 14, 2011; accepted February 20, 2011; posted February 25, 2011 (Doc. ID 140554); published March 28, 2011 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 OCIS codes: 140.3460, 140.3500, 140.3580, 140.3070, 140.3560, 140.3570.
In recent years, Er:YAG lasers have been developed for eye-safe emission at 1645 nm for both cw and Q-switched applications, including remote sensing, range finding, spectroscopy, lidar, and wavelength conversion [1–3]. 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 1 μm (Nd:YAG) [4,5], 2:01 μm (Tm:YAG) [6], and 2:09 μm (Ho:YAG) [7]. In this Letter, we report, to our knowledge, the first demonstration of an Er:YAG NPRO at 1645 nm. 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], 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 14 mm ðwidthÞ × 3 mm ðheightÞ × 13 mm ðlengthÞ 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 30 mm 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 5 cm concave (internally) for a second. The IC/OC of both crystals was coated for high transmission of the 1532 nm p-polarized pump and a reflectivity (R) of 99% for the s-polarized 1645 nm laser output. The high R at 1645 nm was chosen to ensure a low threshold and narrow linewidth, rather than to optimize power and efficiency. 0146-9592/11/071197-03$15.00/0
Additionally, the IC/OC coating has a higher loss (R < 90%) for p-polarized light at 1645 nm 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 0:4 T were stacked as needed. (The Verdet constant for the YAG crystal at 1645 nm is calculated to be ∼6:9 × 10−5 rad=mm-T [9], which is about 2.6 times smaller than at 1 μm. ) As depicted in Fig. 2, the laser crystal was pumped with an IPG cw linearly polarized Er fiber laser at 1532:4 nm, with a spectral linewidth of 0:2 nm (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 1645 nm 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 0:01 nm (1:1 GHz) 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], 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 0:4 T magnet was sufficient to yield a stable unidirectional single frequency up to 0:1 W, whereas four magnets were required for unidirectional operation at 0:5 W. We further observed that stable single-frequency operation also necessitated that the
Fig. 1.
(Color online) Schematic of Er:YAG NPRO crystal.
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Fig. 2. (Color online) Experimental layout for the NPRO pumped by an Er fiber laser.
Fig. 3. (Color online) NPRO single-frequency power performance data comparison: pump beam diameter 120 μm (triangles with blue solid curve) versus 140 μm (squares with red dotted curve). Smaller pump beam diameter yields more efficient output and stable single-frequency operation.
Fig. 4. (Color online) NPRO output mode structures measured by Ando OSA: red dotted curve (multimode) versus blue solid curve (single mode).
Fig. 5. (Color online) NPRO single-mode cw tuning as a function of the crystal temperature. The tuning rate of 0:022 nm=°C is measured.
pump mode diameter be smaller than 170 μm. Optimum performance was achieved when the pump diameter was set to 120 μm, yielding an optical-to-optical efficiency of 11% at 0:45 W 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 0:05 nm mode separation measured with the OSA agreed with the predicted value for the 30 mm 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 1645:214 nm (∼5 GHz), as shown in Fig. 5. The wavelength tuning rate is 0:022 nm=°C (−2:5 GHz=°C). The NPRO output power increased linearly from 189 mW at 15 °C to 198 mW 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 M 2 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 selfheterodyne 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 25 km fiber for a time delay τd of about 125 μs, which is sufficiently longer than the expected laser coherence time. The transmitted beam was frequency shifted by an acousto-optic modulator (AOM) operating at 27:13 MHz. The two beams were recombined by a fiber coupler. The beat signal was detected by an amplified InGaAs photodetector and displayed on an RF spectrum analyzer. The resolution of the delayed selfheterodyne method can be estimated from the expression ½−0:4 logðΔf B τd Þ þ 0:6=τd , where Δf B is the spectral analyzer bandwidth [9]. For our setup, τd ¼ 125 μs and Δf B ¼ 0:5 kHz, which yields a resolution of 8:6 kHz. An RF spectrum of the self-heterodyne signal is shown in Fig. 8 at the NPRO output power 100 mW over a 10 s sweep time. This spectrum shows a central peak and relaxation oscillation sidebands at 146 kHz. The amplitudes of both sidebands are 25 dB below that of the central feature. Relaxation oscillations are common in
Fig. 6. (Color online) NPRO output beam quality M 2 data versus output power. Inset: image of output far-field beam profile.
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Fig. 7. (Color online) Experimental setup for NPRO delayed self-heterodyne linewidth measurement.
most solid-state lasers, occurring when the upper laser level lifetime is much longer than the cavity decay time. The central peak of −3 dB bandwidth in Fig. 8 was measured to be 30 kHz, which corresponds to a deconvolved FWHM linewidth of 21 kHz, assuming a Gaussian lineshape. We also observed that the laser linewidth and relaxation oscillation frequency increased to 26 kHz and 173 kHz, respectively, at a higher laser power level of 150 mW. 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 0:5 W single-frequency Er:YAG NPRO operating at 1645 nm that is resonantly pumped by a Er fiber laser at 1532:4 nm. The device was tunable over a 5 GHz range without mode hopping and had a bandwidth of 21 kHz (over a 10 s interval). The wavelength tuning rate of 0:022 nm=°C (−2:5 GHz=°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 eyesafe emission for various lidar and other remote sensing applications.
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Fig. 8. (Color online) Delayed self-heterodyne signal recorded by an RF spectral analyzer at 100 mW. NPRO single-frequency operation. Full width at −3 dB was measured to be 30 kHz.
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