With the advent of optical frequency combs (OFCs)[
Chinese Optics Letters, Volume. 19, Issue 12, 121405(2021)
Long-term frequency-stabilized optical frequency comb based on a turnkey Ti:sapphire mode-locked laser
We report a long-term frequency-stabilized optical frequency comb at 530–1100 nm based on a turnkey Ti:sapphire mode-locked laser. With the help of a digital controller, turnkey operation is realized for the Ti:sapphire mode-locked laser. Under optimized design of the laser cavity, the laser can be mode-locked over a month, limited by the observation time. The combination of a fast piezo and a slow one inside the Ti:sapphire mode-locked laser allows us to adjust the cavity length with moderate bandwidth and tuning range, enabling robust locking of the repetition rate (fr) to a hydrogen maser. By combining a fast analog feedback to pump current and a slow digital feedback to an intracavity wedge and the pump power of the Ti:sapphire mode-locked laser, the carrier envelope offset frequency (fceo) of the comb is stabilized. We extend the continuous frequency-stabilized time of the Ti:sapphire optical frequency comb to five days. The residual jitters of fr and fceo are 0.08 mHz and 2.5 mHz at 1 s averaging time, respectively, satisfying many applications demanding accuracy and short operation time for optical frequency combs.
1. Introduction
With the advent of optical frequency combs (OFCs)[
The first generation of OFCs used Kerr-lens mode-locked Ti:sapphire lasers[
Although fiber combs have merits of robustness, they are not perfect. (1) In order to obtain enough lasing gain, commercial fiber combs[
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On the contrary, OFCs based on Ti:sapphire mode-locked lasers have moderate repetition rates from hundreds of MHz to 10 GHz[
However, easy operation and long continuous operation are the weakness of Ti:sapphire combs. Normally, Ti:sapphire mode-locked lasers are not easy to get mode-locked. Unfortunately, they lose mode locking easily due to their sensitivity to environmental perturbations, e.g., temperature variation, airflow disturbance, dust contamination, etc. As a result, it is difficult to keep a Ti:sapphire comb mode-locked for more than one day. Secondly, actuators to phase lock and have limited servo bandwidth and tuning range, which cannot sustain environmental perturbations.
In this Letter, to solve these problems, firstly, the cavity of the Ti:sapphire mode-locked laser is designed to have low dust contamination and small light misalignment, enabling the Ti:sapphire laser to keep mode-locked for more than a month, limited by the observation time. In order to stabilize for a long time, both a fast and a slow piezo-transducers (PZTs) are employed as actuators to compensate the fluctuation and drift of the cavity length. By combining a fast analog feedback to the pump current of the Ti:sapphire mode-locked laser and a slow digital feedback to an intracavity wedge and the pump power of the Ti:sapphire laser, is stabilized robustly. After taking the above measures, both and of the Ti:sapphire OFC are stabilized to a hydrogen maser (denoted H maser) for 5 days, a significant step towards long-term operation compared with previous Ti:sapphire combs. The frequency instability of phase-locked and is 0.08 mHz and 2.5 mHz at 1 s averaging time, respectively. Such a continuously frequency-stabilized Ti:sapphire OFC is believed to be a reliable and accurate tool in precision measurement.
2. Experimental Setup
The mode-locked laser used in this paper is a commercial Ti:sapphire mode-locked laser pumped by a 532 nm solid-state laser (Laser Quantum, Taccor 10). The laser cavity is a ring cavity, which consists of six mirrors, as shown in Fig. 1. Five of the cavity mirrors are installed in fixed mirror mounts without adjusting knobs in order to avoid cavity misalignment, and one cavity mirror (lower left) is mounted in a motorized mirror mount. The alignment of the laser cavity is optimized with the motorized mirror mount by monitoring the laser output power. The Ti:sapphire mode-locked laser achieves automatic mode locking by controlling a motor to knock one of the cavity mirrors (lower right). As long as it is mode-locked, it outputs a pulse train with an average power of more than 2 W and a pulse duration of 30 fs. To extend the mode-locked time, the laser cavity is sealed and circulated with filtered air, preventing dust contamination on cavity mirrors and the Ti:sapphire crystal. Moreover, the Ti:sapphire crystal is shifted once in 200 h to ensure that the crystal is in excellent status. The base plate of the Ti:sapphire mode-locked laser is temperature-controlled at 23.5°C, eliminating cavity misalignment due to temperature fluctuation. Benefitting from the special laser design, the Ti:sapphire laser can keep continuously mode-locked for over a month, limited by our observation time.
Figure 1.Experimental setup of the frequency-stabilized Ti:sapphire OFC. The solid lines represent the light path, while the dashed lines represent the electrical path. PPKTP, periodically poled KTiOPO4 crystal; OBPF, optical bandpass filter; PCF, photonic crystal fiber; PD, photo detector.
To fully stabilize each comb line, two degrees of freedom need to be stabilized, and . The detection and frequency stabilizations of and are shown in Fig. 2.
The repetition rate, , is directly detected on the photo detector () with an SNR of 70 dB under a resolution bandwidth (RBW) of 300 kHz. The SNR of is stable from day to day. Figure 2(a) shows the frequency drift of in free running. It fluctuates within 4 kHz in 4 days, which is mainly related to the room temperature fluctuation. The frequency instability of is at 1 s averaging time, which is introduced by noise at the frequency of a few hertz, i.e., the vibration noise and air perturbation.
Figure 2.(a) Frequency fluctuation of fr when it is free running. (b) fceo detected at 532 nm with an RBW of 300 kHz. (c) Optical beat signals of the Ti:sapphire comb against a cavity-stabilized laser at 1064 nm (in dark wine), a 729 nm laser (Ca+ clock, in magenta), a 698 nm laser (Sr clock, in red), a 578 nm laser (Yb clock, in yellow), and an 532 nm laser (iodine optical reference, in green) with an RBW of 300 kHz.
The signal of (nearly 1 GHz) is mixed on a double-balanced mixer (DBM) against a radio frequency signal at a frequency of synthesized from an H maser. The output of the DBM (error signal) is low-pass filtered and sent to a servo. To achieve long-term and robust frequency stabilization, we employ a fast and a slow servo loop to stabilize . The fast servo controls the fast PZT to compensate the rapid fluctuations of the cavity length. The fast servo provides a servo bandwidth of more than 50 kHz but a limited servo range of . To compensate slow drift of the cavity length due to environmental temperature drift, the slow servo integrates the error signal with a time constant of 100 s to control the slow PZT, which provides a tuning range as large as .
To fully stabilize the OFC, is also detected and stabilized. The pulse train of the Ti:sapphire mode-locked laser is dispersion-compensated by chirped mirrors (not shown in Fig. 1) before focusing into two pieces of PCF (FemtoWHITE 800) for spectrum broadening to more than one octave, e.g., 530–1100 nm.
The broadened spectrum output from one of the PCFs (not shown in Fig. 1) allows us to obtain beat-notes against optical atomic clocks and c.w. lasers with an SNR more than 40 dB (). Figure 2(c) shows the beating signals against a cavity-stabilized laser at 1064 nm[
The broadened spectrum output from the other PCF allows us to detect the signal of near 140 MHz in a collinear interferometer[
3. Methods and Results
To extend the tuning range of , we design a digital servo of to compensate its long-term drift by adjusting the intracavity dispersion via an intracavity wedge and the pump power of the Ti:sapphire mode-locked laser. The logical block diagram is shown in Fig. 3. In this digital servo, the output voltage () of the fast servo for is read once a second on a digital multimeter (Keithley 2000) to a computer. When is larger than a preset value , the computer will send a command to adjust the pump power by a step of 1 mW. The value of is preset to 30 mV, slightly smaller than the maximum input voltage of the Ti:sapphire laser pump current. However, when the change of the pump power is accumulated by more than 50 mW, there will be undesirable noise appearing on . For this reason, when the pump power is changed by an accumulated mount larger than ( is set as 50 mW here), the computer will precisely move the wedge inside the laser cavity and adjust the pump power back to its initial value (). With this additional digital slow servo, the output voltage of the fast servo is kept in a range of , and can keep stabilized for a few days.
Figure 3.Logic block diagram of digital servo of fceo.
A multi-channel frequency counter (K + K Messtechnik GmbH) is employed to measure and when they are phase-locked to the H maser. The counter with a gate time of 1 s shares the same H maser time base. Both signals are mixed down to 1 MHz to achieve a better frequency counting resolution.
The total locking period of the comb is about 5 days, as shown in Figs. 4(a) and 4(b). By the end of the fifth day, the room temperature was raised by 3 K, leading to a large length change of the slow PZT in order to compensate the cavity length change. This made the locking state of get worse (stay frequency-stabilized but with more frequency jumps) due to the coupling between the tuning of and that of .
Figure 4.Frequency jitters of (a) fceo and (b) fr when phase-locked to an H maser. (c) The frequency instability of fceo (purple dots) and fr (black squares).
During the locking period, the position of the wedge was adjusted 12 times, and a total of 27 data points of are removed from Fig. 4(a) since a cycle slip of happened when the wedge was adjusted. The wedge is installed on a step motor, which tunes with a minimal step of in a short time, leading to the cycle slip of . The frequency instabilities of phase-locked and , as shown in Fig. 4(c), are 2.5 mHz and 0.08 mHz at 1 s averaging time, respectively. The results demonstrate that the OFC based on the Ti:sapphire mode-locked laser can be frequency-stabilized over 5 days. Although the relative frequency instability of each comb line is only at 1 s averaging time, limited by the H maser, this Ti:sapphire comb can support coherence transfer or optical frequency division with an additional frequency instability of at 1 s averaging time and an uncertainty at the level[
In the near future, the OFC based on the Ti:sapphire laser will be improved to achieve longer frequency stabilization time. Firstly, the OFC could be enclosed in a temperature-controlled chamber to protect from room temperature fluctuation. Secondly, in order to avoid a cycle slip of , the step motor used to tune the wedge position could be replaced by a PZT to adjust smoothly. Thirdly, the coupling issue between the tuning of and that of still needs to be solved in the following study.
4. Conclusion
In this paper, we demonstrate a turnkey Ti:sapphire mode-locked laser with continuous mode-locking time over a month. By employing both fast actuators for wide servo bandwidth and slow actuators for large tuning range to control and , the OFC based on the Ti:sapphire mode-locked laser can be continuously frequency-stabilized over 5 days. Such a frequency-stabilized Ti:sapphire OFC is believed to be a reliable and accurate tool in precision measurement.
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Guang Yang, Haosen Shi, Yuan Yao, Hongfu Yu, Yanyi Jiang, Albrecht Bartels, Longsheng Ma, "Long-term frequency-stabilized optical frequency comb based on a turnkey Ti:sapphire mode-locked laser," Chin. Opt. Lett. 19, 121405 (2021)
Category: Lasers, Optical Amplifiers, and Laser Optics
Received: Apr. 26, 2021
Accepted: Jul. 7, 2021
Published Online: Sep. 17, 2021
The Author Email: Haosen Shi (hsshi@lps.ecnu.edu.cn), Yuan Yao (yyao@lps.ecnu.edu.cn)