Methods and evaluation of frequency aging in distributed-feedback laser diodes for rubidium atomic clocks

Abstract

Distributed-feedback laser diodes emitting at 780 nm have been evaluated, with respect to the aging of the injection current required for reaching the rubidium D2 resonance line. Results obtained for lasers operating in air and in vacuum for 9 months are reported. When operated at constant temperature, the laser current required for emission at the wavelength of the desired atomic resonance is found to decrease by 50 to 80 μA per month. The impact of this result on the lifetime and long-term performances of laser-pumped rubidium atomic clocks is discussed.Reliable single-mode and narrowband laser diodes are required for a wide range of applications and precision instrumentation including atomic clocks [1][2][3], magnetometers [4], precision gyroscopes [5], trace gas sensing [6], and telecommunication. The reliability and lifetime of such instruments critically relies on the same characteristics of the laser diodes implemented [7,8]. A variety of tests were established [9] and are now routinely performed on laser diodes, both for applications on ground [10] and in space [11,12].Standard procedures on laser reliability testing include the laser linewidth and the intensity noise [9], but other spectral properties parameters relevant for high-precision instrumentation using narrow atomic resonances, such as atomic clocks and gyroscopes, are not considered, e.g., frequency noise or aging of the injection current required to reach a specific wavelength. Here we report on our studies on the aging of the current at resonance for distributed-feedback (DFB) laser diodes, which emission at 780 nm corresponds to the Rb D2 transition, as used in Rb atomic clocks. This current is understood as the injection current (at constant laser chip temperature) needed to maintain the laser emission precisely at the center of the atomic resonance. The aging of this current, or frequency aging, can limit the spectral lifetime of the laser and finally also the instrument lifetime, because at some time it can become impossible to maintain the Rb resonance wavelength by simply adjusting the laser current. Measurement of the laser wavelength throughout the aging procedure was previously applied to a 780 nm vertical-cavity surface-emitting laser (VCSEL) [7] and in radiation tests on DFB at 1:55 μm [11], but two-point measurements (before and after aging) are predominant in the literature [12][13][14]. In this Letter, we report on aging of the current at resonance, measured several times per day over many months.The lasers under study are GaAs DFB laser diodes (Eagleyard Photonics, EYP-DFB-0780-00080-1500) [15]. They emit at 780 nm. Three devices were evaluated on the long-term. Two were packaged in a TO-9 can style with integrated monitor diode, while the third one was in a TO-3 housing with a thermoelectric cooler (TEC) and thermistor (NTC) additionally integrated.Two different methods were applied to evaluate the frequency aging of the devices; both imply an Rb D2 transition at 780 nm as frequency discriminator. In the first method, the las...

Compact and frequency stabilized laser heads for Rubidium atomic clocks

“…This parameter is to be understood as the change -with time -of the injection current required for the laser to keep a fixed wavelength when operated at constant temperature. For DFB lasers emitting at a wavelength of 780 nm corresponding to the Rb D2 transition, as used in Rb atomic clocks, the laser current at resonance has been found to decrease by 50 to 80 A per month (-0.6 to -1 mA/year) [11]. Such aging has been measured with a frequency-stabilized laser head similar to the ones reported here, where the injection current is continuously adjusted by a feedback loop to maintain the laser frequency at the center of a sub-Doppler line, while the laser temperature is maintained constant.…”

Pulsed optically pumped rubidium clock with high frequency-stability performance

“…These lasers show stable, narrow-band single-mode emission while using only one pump-current section. For this laser type, aging of the emission wavelength is reported to be slow enough to allow for 15 or 20 years of clock operation [16]. at the Fourier frequency f = 300 hz, we measured a relative intensity noise (rIn) of 2 × 10 −14 /hz and a frequency noise of 4 khz/hz 1/2 .…”

Frequency Ageing and Noise Evolution in a Distributed Feedback Quantum Cascade Laser Measured Over a Two-Month Period

“…The wavelength shift of a 780-nm vertical-cavity surface-emitting laser (VCSEL) was measured during an ageing process [2] and a similar measurement was realized during the radiation test of a 1.55-μm distributed feedback (DFB) laser [3]. The drift of the laser current at resonance of an Rb vapor cell transition has also been studied over several months in various DFB lasers at 780 nm, both at atmospheric conditions and under vacuum [4]. These studies have been realized so far with near-infrared laser diodes, but no similar investigation has been performed in mid-infrared lasers.…”