Proceedings Article | 4 March 2019
KEYWORDS: Global Positioning System, Quartz, Oscillators, Crystals, Terahertz radiation, Laser stabilization, Diode pumped solid state lasers, Waveguides, Semiconductor lasers, Satellite navigation systems
As reported at Photonics West last year, diode pumped solid state (DPSS) green laser pointer modules (GLP) can have surprisingly good spectral characteristics and operate with a near single longitudinal-mode and narrow linewidths (approx. 30 kHz). Linewidths are measured by beat-notes against a kHz linewidth DPSS NPRO laser. Here we report that it is possible to phase-lock the optical frequency of the GLP to a self-referenced fs-optical-frequency-comb. This allows us to both know and control the absolute optical frequency of the GPL relative to a high-stability quartz-crystal oscillator that is steered to GPS signals. GPS and other GNSS signals are available everywhere and provide excellent frequency accuracy from the atomic clocks in space that are in turn steered by the elaborate world-wide GNSS infra-structure and ultimately referenced to high-accuracy primary atomic frequency standards.
GPS-steered (disciplined) quartz crystal oscillators are readily available from many sources and provide a simple route to fractional frequency instability of about 1x10-12 for τ ≲ 1000 s and on longer time scales the frequency knowledge improves as roughly 1/ τ reaching the 10-14 range at 105 s. A fs-comb can transfer that stability and accuracy to the optical region with high fidelity.
Characterization of GLP
The DPSS GLP are particularly interesting and convenient. They provide excellent power (10’s of mW) at 532 nm and 1064 nm, with near-TEM00 spatial-mode. With some care in controlling operating conditions GLP can provide nearly signal-longitudinal-mode operation with a narrow-linewidth, and the optical frequency can be stabilized with simple control systems to iodine saturated-absorption signals or as described here GNSS-GPS frequency references. Typical modern GLP are based upon an 808 nm diode laser that pumps a short NdYVO4 gain chip that lases at 1064 nm and is directly bonded to a KTP crystal that doubles the frequency to 532 nm.
However, the compact GLP also have some limitations, including: regions of operating parameters (temperature, pump-laser power and spectral characteristics) where the GLP operates on several modes (1 to > 5), regions that have significant instability and noisy (AM and FM). Some regions of the parameter space also show large relaxation oscillations at low frequencies (e.g. near 1 MHz). So far, we have not done a statistically significant study of the performance characteristics of the GLP. However, we find that the majority of the GLPs that we have tested have usefully large regions of temperature and pump-laser current where the GLP will run reliably on a dominant single longitudinal-mode for long periods of time (about two years is our longest test to date).
Approach – GPS steering of visible lasers.
The basic approach is: Received GPS signals steer the frequency of a high-stability quartz crystal oscillator that is used to lock the repetition rate (frep) and offset frequency (fceo) of the fs-comb. In our current experiments we use a commercial self-referenced fs-optical frequency comb centered at 1560 nm that is spectrally broadened in highly nonlinear (HNL) optical fiber to cover the spectral range from about 1000 nm to 2000nm. Beatnotes between the super-continuum from the fs-comb and the cw GLP are done at 1064 nm. To extract the 1064 nm from the GLP we removed the filter that blocks the 1064 nm output from the NdYVO4-KTP resonator. [Note: We find that in some low cost GLP significant (10’s of mW) 1064 nm already leaks out along with the 532 nm.] A beatnote between the 1064 nm output from the GLP and a mode of the broadened super-continuum from the fs-comb is then processed in a phase-frequency detector circuit to generate an error-signal that is used to control the optical frequency of the GLP.
Course tuning of the GLP frequency is done with temperature combined with the DC set point of the injection current of the pump laser. Frequency stabilization is achieved via the pump-laser injection current, which changes the 808 nm pump-power and tunes the frequency of the NdYVO4 laser by changes in the gain and temperature within the lasing mode. For normal operating conditions, we measure an effective frequency response bandwidth (3 dB) of roughly1 kHz for pump laser control of the GLP optical frequency. Because the intrinsic frequency stability of the GLP is quite good (measured spectral linewidth ≈ 30 kHz) the frequency control via pump power is sufficient to achieve a phase-lock to the self-referenced fs-comb. The residual fractional frequency instability in the optical phase-lock is measured to be approximately 2 x10-14/ τ , for averaging time τ in seconds. This is well below the instability of even the most stable high quartz crystal oscillators, (in our case the quartz instability is slightly less than 1x10-12 for τ < 300s.
On short time scales ≲ 100 ms the GLP has better stability and narrower linewidth than the modes of the super-continuum from the GPS steered self-referenced fs-comb. That is because the intrinsic stability of the GLP is better than the phase-noise of even a high-quality quartz crystal when multiplied up to 500 THz. The multiplication to optical frequencies produces comb-mode linewidths of about 300 kHz to 2 MHz.
With our sincere thanks for the loan of a Si3N4 waveguide chip from NIST, we have also successfully detected good quality beatnotes at 1064 nm between the GLPs and the remarkable super-continuum that is generated by the Si3N4 waveguides. Those beatnotes have also been used to phase-lock the GLP and to detect fceo. However, currently our best performance comes from the supercontinuum from the HNL fiber. Nonetheless, given the very broad super-continuum that can be generated by Si3N4 waveguides it should be feasible to use GPS, as done here, to control the frequency of visible lasers with wavelengths ranging from 400 nm and 2000
Performance Results
The net result of the approach reported here is GPS-steered GLP at 532 nm (563 THz) and 1064 nm (281 THz) that provides 10’s of mW of output power in the green and IR, with good spatial modes, dominantly single-longitudinal-mode, with a linewidth of ≈ 30 kHz, and center frequency stabilized to 500 Hz (σy(τ) ≤ 1x10-12 for averaging times ≳ 30 ms). The absolute frequency is known to 12 digits when locked to GPS. [We note that, at least in principle, the GLP could stay locked and steered to GPS for long durations, in which case commercial GPS-steered quartz oscillators can provide long-term frequency stability that averages down into the 10-14 range, (corresponding to 5 Hz at 500 THz)].
Achieving optical phase-lock ensures that the 563 THz (532 nm) frequency is stabilized relative to the GPS reference, but, as usual, to know the absolute optical frequency requires determination of the large integer N of the fs-optical comb mode (comb mode frequency fN = Nfrep + fceo ). In our case, the pulse repetition frequency frep= 250 MHz and the carrier envelope offset frequency fceo=20 MHz). N can be determined by measuring the cw laser wavelength/frequency using a wavemeter that has six digits of accuracy, or by keeping the cw laser stable and changing frep sufficiently to determine N, or in many cases of interest in AMO science, atomic/molecular spectra can provide sufficient knowledge of N. In the case of GLP it is sometime feasible to use molecular iodine spectra to determine N. The frequency instability and frequency accuracy of the GPS-steered-quartz and subsequently the locked fs-comb are verified in our lab by independent and long-term measurements against Cs and Rb atomic frequency standards, and also previously against Sr-optical and H-maser frequency references.
GLPs provide convenient, bright green 532 nm, and 1064 nm near IR, and if frequency stabilized can serve as optical frequency and wavelength references for numerous applications. When steered to GPS these lasers can provides 12 digits of frequency accuracy and a short-term spectral linewidth of about 30 kHz. That performance exceeds the requirements of most real-world applications of frequency-stabilized lasers (e.g. calibration of spectrometers, references for interferometers, dimensional metrology, coordinate measuring and position control systems, references for atomic and molecular spectroscopy, etc.).
With increasing availability and simplification of fiber-based mode-locked lasers that can be “self-referenced” the approach outlined here becomes an attractive method for absolute frequency stabilization of multiple laser frequencies that are typical used in atomic physics and cold-atom experiments (e.g. in our lab for Yb, I2, Rb, Cs, etc.)
Acknowledgments. This research was supported in-part by the DARPA-ACES program. We also thank A. Cable, D. Carlson, and S. Papp for important contributions to this project.
References
i) e.g. green laser pointer module, https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=5597
ii) Websites of: MicroSemi, FEI-/Zyfer, Stanford Research, EndRun Technologies, etc.
iii ) Menlo Systems, Er-fiber based fs comb.
