Soumya Sarang

Intrinsically stable high-power single longitudinal mode laser using spatial hole burning free gain

A fundamental advantage of lasers is their ability to produce a large number of photons in a single optical mode, yet this is achieved in only a minor fraction of devices due to the instability mechanism called spatial hole burning. Here, we exploit the spatial hole burning free nature of a stimulated scattering gain medium to demonstrate single longitudinal mode (SLM) operation in a generic standing wave cavity. A continuous wave diamond Raman oscillator with multi-Watt-level output power and a frequency stability of 80 MHz is demonstrated without use of additional mode-selective elements. Mode stability is addressed by considering the coupling of the Stokes power with thermally induced optical path length changes in the gain medium. The results foreshadow a novel approach for greatly extending the power and wavelength range of SLM laser sources, and with potential advantages for achieving sub-Poissonian intensity noise and sub-Schawlow–Townes linewidths.

Birefringence and piezo-Raman analysis of single crystal CVD diamond and effects on Raman laser performance

Defect-induced stress has been mapped in optical-grade synthetic diamond (chemical vapor deposition grown, low nitrogen, low birefringence) using Metripol polarimetry, Mueller polarimetry, and Raman microscopy. Large circular retardance was observed in the 8 mm long ⟨110⟩ cut crystal with values up to 28° for some paths along the major axis. Metripol-determined values for linear birefringence magnitude and fast-axis direction in such regions have significant error. Stress-induced shifts in Raman frequency were observed up to 0.7  cm−1, which we deduce result from uniaxial and biaxial stresses up to 0.86 GPa. We also elucidate the effect of stress on diamond Raman laser performance. For high cavity Q Raman lasers, the direction of the linear birefringence axis is found to be a primary factor determining the laser threshold and the input–output polarization characteristics.

Single longitudinal mode diamond Raman laser in the eye-safe spectral region for water vapor detection

We report a narrowband and tunable diamond Raman laser generating eye-safe radiation suitable for water vapor detection. Frequency conversion of a tunable pump laser operating from 1063 to 1066 nm to the second order Stokes component in an external standing-wave cavity yielded 7 W of multimode output power in the wavelength range from 1483 to 1488 nm at a conversion efficiency of 21%. Stable single longitudinal mode operation was achieved over the whole tuning range at low power (0.1 W), whereas incorporation of a volume Bragg grating as an output coupler enabled much higher stable power to be attained (0.5 W). A frequency stability of 40 MHz was obtained over a minute without active cavity stabilization. It was found that mode stability is aided via seeding of the second Stokes by four-wave mixing, which leads to a doubling of the mode-hopping interval. The laser was employed for the detection of water vapor in ambient air, demonstrating its potential for remote sensing applications.

Large brightness enhancement for quasi-continuous beams by diamond Raman laser conversion

High average power lasers with high beam quality are critical for emerging applications in industry and research for defense, materials processing, and space applications. However, overcoming thermal effects in the gain medium remains the key challenge for increasing laser brightness at high powers. Here we report a means for increasing the beam brightness of high-power continuous-wave (CW) beams based on external cavity Raman lasers using diamond, a material with thermal properties far superior to any other laser material. With pump beam quality in the range M2=2.3-7.3, efficient pump-limited conversion to an M2=1.1 Stokes beam is achieved in all cases, with increases in brightness from the pump by factors as high as 12.7. The influence of pump beam quality on laser threshold and slope efficiency is analyzed. This Letter foreshadows an alternative approach for scaling the brightness of CW lasers using high-power, moderate beam quality pumps up to M2=20 or more, such as thin-disk and slab lasers and fiber lasers operating in a mode instability regime.

302 W quasi-continuous cascaded diamond Raman laser at 1.5 microns with large brightness enhancement

We report a second Stokes diamond Raman laser at 1.49 μm capable of high power and large-scale-factor brightness enhancement. Using a quasi-continuous 1.06 μm pump of power 823 W (0.85% duty cycle) and M2 up to 6.4, a maximum output power of 302 W was obtained with an M2 = 1.1 providing an overall brightness enhancement factor of 6.0. The pulse length of ~210 μs was selected to ensure operation was representative of steady-state continuous lasing conditions in the diamond bulk. Accompanying theoretical calculations indicate that even more strongly aberrated pumps may be used to efficiently generate high beam quality output and with higher brightness enhancement factors. This diamond-based beam conversion technique addresses needs for high brightness and efficient eye-safe sources using low-brightness 1 μm pumps and reveals a widely-applicable route to practical high brightness lasers of increased wavelength range.

Single-longitudinal-mode diamond Raman lasers in the near-infrared spectral region

Lasers operating in single-longitudinal-mode (SLM) are of great importance for high-precision measurements in nonlinear optics and spectroscopy as well as for applications in remote sensing, laser cooling and the flourishing field of gravitational wave detection. However, stable SLM in standing-wave inversion lasers is impeded by spatial hole burning which causes mode instability and can only be overcome at the expense of power limitations and/or higher complexity of the laser system, e.g. by means of injection-seeding, ring or microchip laser designs. As an alternative approach, we demonstrate that the nonlinear optical process of stimulated Raman scattering provides a spatial hole burning free gain which enables the generation of SLM output that is intrinsically stable [1]. The underlying mechanism was harnessed for the development of two compact Raman laser configurations which were realized as external standing-wave cavities, without use of any mode-selective elements, and containing only the CVD diamond Raman-active gain medium. Efficient frequency conversion of a tunable Yb-fiber-amplified distributed feedback (DFB) laser emitting around 1064 nm to the first-and second-order Stokes components produced SLM output in the near-infrared spectral region at powers up to 7 W, while wavelength tuning over a range of 700 GHz was accomplished by varying the temperature of the DFB pump laser, as depicted in Fig. 1a.

High power single-longitudinal-mode diamond laser using Hänsch-Couillaud-type stabilization

We report a cavity stabilization scheme to enable single longitudinal mode operation from a diamond Raman laser at increased power. An output power of 7.2 W is demonstrated using a simple standing-wave laser cavity without the addition of frequency-selection elements.

Brightness enhancement of continuous-wave beams using a diamond Raman laser

Brightness enhancement of high power quasi-continuous-wave beams is investigated in a diamond Raman laser as a function of the input beam quality. Up to 389 W of 1240 nm output is generated with a 78.2% linear slope efficiency and 52.5% overall conversation efficiency. For an M2 = 3.3 pump, the output beam quality factor of M2 = 1.08 was obtained, yielding a brightness enhancement factor of 3.7. Higher brightness enhancement factors are predicted when using lower beam quality pumps provided that a minimum input brightness is attained. The results point to new ways of creating brighter laser beams from high power lasers having degraded beam quality.

High-brightness and narrow-linewidth diamond Raman lasers

We present our recent advances in the field of Raman frequency conversion using high-optical quality CVD-diamond. Different diamond Raman lasers were developed for efficiently generating multi-Watt output at specific wavelengths from the visible to the eye-safe spectral range, while single-frequency operation was accomplished by exploiting an intrinsic mode stability mechanism.

High-gain 87 cm-1 Raman line of KYW and its impact on continuous-wave Raman laser operation

We report a quasi-continuous-wave external cavity Raman laser based on potassium yttrium tungstate (KYW). Laser output efficiency and spectrum are severely affected by the presence of high gain Raman modes of low frequency (< 250 cm-1) that are characteristic of this crystal class. Output spectra contained frequency combs spaced by the low frequency modes but with the overall pump-to-Stokes conversion efficiency at least an order of magnitude lower than that typically obtained in other crystal Raman lasers. We elucidate the primary factors affecting laser performance by measuring the Raman gain coefficients of the low energy modes and numerically modeling the cascading dynamics. For a pump polarization aligned to the Ng crystallo-optic axis, the 87 cm-1 Raman mode has a gain coefficient of 9.2 cm/GW at 1064 nm and a dephasing time T2 = 9.6 ps, which are both notably higher than for the 765 cm-1 mode usually considered to be the prominent Raman mode of KYW. The implications for continuous-wave Raman laser design and the possible advantages for applications are discussed.