Cite this Article
Dai Shu-Tao, Wu Hong-Chun, Shi Fei, Deng Jing, Ge Yan, Weng Wen, Lin Wen-Xiong. Development of an injection-seeded single-frequency laser by using the phase modulated technique. Chinese Physics B, 2018, 27(5): 054212  
Permissions
Development of an injection-seeded single-frequency laser by using the phase modulated technique
Dai Shu-Tao1, 2, Wu Hong-Chun1, Shi Fei1, Deng Jing1, Ge Yan1, Weng Wen1, Lin Wen-Xiong1, 2, †
Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China
University of Chinese Academy of Sciences, Beijing 100049, China

 

† Corresponding author. E-mail: wxlin@fjirsm.ac.cn

Abstract

An injection-seeded single-frequency Q-switched Nd:YAG laser is accomplished by using a phase modulated ramp-fire technique. A RbTiOPO4 (RTP) electro-optic crystal is selected for effective optical path length modulation of the slave self-filtering unstable resonator. This single-frequency laser is capable of producing 50 mJ pulse energy at 1 Hz repetition rate with a pulse width of 16 ns. The standard deviation of laser pulse intensity for consecutive 100 shots from the mean pulse intensity is less than 1.05%. A spectral linewidth of less than 0.5 pm with a frequency jitter of about 14 fm over 30 min is obtained.

PACS: 42.60.Gd;42.60.Da;42.60.By
Keyword:single-frequency Q-switched Nd:YAG laser;injection seeding;phase modulation;self-filtering unstable resonator (SFUR)
1. Introduction

All-solid-state Q-switched single-frequency lasers are widely used in many fields, such as nonlinear optics, high-resolution spectroscopy and Doppler lidar.[1–4] Currently, injection seeding is one of the most frequently used techniques to produce single-axial-mode output from a Q-switched laser. It is achieved by injecting a low power narrow linewidth single-frequency seed laser into a Q-switched slave oscillator. If the seed laser has sufficient power, in the absence of spatial hole burning, the mode competition in the slave cavity will establish single-axial-mode oscillation.[5] However, the slave cavity must be in resonance with the frequency of the seed when the Q-switch of the slave cavity is activated. Hence, an active feedback locking system is required to ensure the resonance.

Schmitt and Rahn described a stabilization feedback locking technique by minimizing the Q-switched pulse build-up time (MBUT).[6] However, the feedback signal was obtained only once after each laser shot. This led to difficulties in certain situations such as low repetition rate operation and high noise environments. Fry et al. reported a ramp-fire locking technique for maintaining a single-frequency operation in severe environments based on fast resonance detection.[7,8] In this approach, optimum axial mode locking was achieved for each laser shot. This became the most reliable approach to realizing the stable single frequency operation. However, these techniques generally rely on a piezoelectric transducer (PZT) mounted on a cavity mirror to modulate the length of the cavity. However, the low time response of PZT limits the response rate of ramp system and the repetition rate of laser operation. The moving of the cavity mirror also results in unstability of the resonator. Zhang et al. demonstrated single-frequency output using a LiNbO3(LN) crystal instead of PZT.[9] They obtained better spectrum performance by adopting LN compared with that by adopting the PZT.

In this work, we apply this intracavity phase modulation technique to a flashlamp-pumped Nd:YAG self-filtering unstable resonator (SFUR). There are several benefits in using the SFUR resonator. The SFUR resonator makes it easier for the laser to achieve seed injection mode matching and large mode volume. Finally, we obtain a high-energy, near-diffraction-limited TEM00 mode and stable narrow linewidth single-axial-mode pulse output.

2. Experiment design and setup

The geometry of the single-frequency laser is shown in Fig. 1. It is composed of a single-frequency seed laser, A Q-switched slave oscillator and a CPLD control circuit.

Fig. 1. (color online) Schematic diagram of The injection-seeded self-filtering unstable resonator.

The seed laser is a commercial 1064 nm single-frequency laser (Mephisto S200NE, Coherent) with a spectral linewidth on the order of 1 kHz and maximum output power of 200 mW. The output light is elliptically polarized, and is transformed into linear polarization state by a quarter-wave and a half-wave plate. A Faraday isolator (IO-5-1064-VHP, Thorlabs) with an isolation of 40 dB is inserted behind the seed laser to prevent feedback from damaging by the Q-switched pulse.

The Q-switched slave oscillator is a flashlamp-pumped self-filtering unstable resonator (SFUR) Nd:YAG laser.[10] The SFUR configuration is a confocal negative branch unstable resonator. A field-limiting pinhole is located on the common focal plane of the cavity mirrors. The aperture is chosen such that a plane wave incident on it is focused by mirror M1 on an Airy disk having the same diameter. Then this will result in the removal of the hot spot and the smoothing of the spatial profile. This yields

where a is the aperture radius, f1 is the focus of M1 and λ is the laser wavelength. The Airy disk is only allowed to propagate beyond the aperture, and on reflection from the mirror M2 it is magnified, collimated and presents a Fourier transformed output.[11]

A RbTiOPO4 (RTP) crystal is used for intracavity optical path length modulation. The Y-cut RTP crystal has dimensions of 3(Z)×3(X)×20(Y) mm3. The Z faces are plated with Ti electrodes onto which the modulating electric field is applied. The polarization of the input seed is parallel to the X-axis. In this case, the double pass effective optical path length change is given by

where L is the crystal length, d is the crystal thickness in the Z direction, nz is the Z-axis refractive index and γ33 is the available electro-optic coefficient. Assume L = 20 mm, d = 3 mm, and γ33 = 35 pm/V,[12] and will be about for an applied voltage of 760 V. Then the seed transmitting the cavity will experience at least one resonant peak as the slave oscillator resonates with the seed laser. The interference signal is monitored by a photodiode detector (PDA10CS, Thorlabs) as shown in Fig. 2. After an appropriate time delay of the flashlamp pumping, the ramp voltage is triggered and applied to the RTP crystal. The interference signal is detected by monitoring the transmitted seed beam through the cavity mirror M1 with a photodiode. The photodiode signal is sent to the CPLD circuit for detecting the peak and generating the Q-switched trigger signal.

Fig. 2. (color online) Ramp voltage and interference signals with ramp-hold-fire technique.
3. Results and discussion

Figure 3 shows the variations of output energy from the laser with pump energy in the cases with and without seed injection. The pulse energy is reduced by 16% in the case with seed injection. This phenomenon due to inhomogeneous saturation gain broadening has been used as a new mode-locking technique called energy reduction (ER) by our group.[13]

Fig. 3. (color online) Plots of output energy versus pump energy in the cases of the laser seeded and unseeded injection.

The beam spatial profile is shown in Fig. 4. A light divergence of 0.1 mrad is achieved. The SFUR configuration automatically produces a collimated output beam and smooth spatial profile. This is beneficial for the next amplifier, harmonic and optical parametric oscillator.

Fig. 4. (color online) Beam spatial profile of the laser.

The temporal shapes of the output pulse with seeded and unseeded injection are shown in Fig. 5. The pulse shape is measured by using a 1 ns rise time InGaAs photodiode (DET10A/M, Thorlabs) and a 1 GHz bandwidth oscilloscope (TELEDYNE MSO104MXs-B, Lecroy). The unseeded pulse shape shows strong modulation due to multimode beating. But the seeded pulse shows a smooth profile as expected for single-frequency operation. The build-up time of the seeded laser is reduced by 27.8 ns. The measured pulse width (FWHM) is around 16 ns at 50 mJ/pulse output energy.

Fig. 5. (color online) Temporal profiles of the laser in seeded and unseeded operation, respectively.

The pulse energy stability is measured by measuring the area of the temporal profile of the laser pulse with the built-in function of the oscilloscope. The results shown in Fig. 6 indicate that the standard deviation (stdDev) of laser pulse intensity for consecutive 100 shots is less than 1.05% of the mean pulse intensity. The pulse width jitter is 183 ps (stdDev).

Fig. 6. (color online) Measured pulse energy and width of laser by oscilloscope.

The frequency spectrum of the seeded injection laser is further analyzed with a commercial Fizeau wavemeter (Angstrom WS/7, HighFinesse) as shown in Fig. 7. The linewidth is measured to be smaller than 0.5 pm, limited by the resolution of the measuring instrument. The wavelength is 1064.42840 nm. The frequency stability is also recorded by a wavemeter over 30 min, which is illustrated in Fig. 8. The frequency jitter is limited to 14 fm (stdDev). As a comparison, the frequency stability over 2 min is illustrated in the inset of Fig. 8 with 1.6 fm (stdDev).

Fig. 7. (color online) Fizeau wavemeter interference pattern for the seeded injection laser. Upper and lower traces correspond to low and high-resolution Fizeau wedges, respectively. The horizontal axis shows the position of the interference signal (arb. units) and the vertical axis indicates relative intensity of the signal (arb. units).
Fig. 8. (color online) Time behavior of the long-term frequency fluctuations recorded shot-to-shot over 30 min. The horizontal axis shows measurement time (min) and the vertical axis indicates measured wavelength (nm). The inset shows the same time behavior for two minutes.

However, the disadvantage of this laser is that the Q-switched firing time jumps randomly in one resonant cycle during the ramp, as shown in Fig. 9. Thus pulse-to-pulse fluctuation due to this jump results in the difficulty of synchronization and pulse energy fluctuation. Taking the advantage of fast no-ringing effect of the RTP crystal intracavity phase modulator, we demonstrate a modified ramp-hold-fire seed injection technique which can well solve this problem.

Fig. 9. (color online) Q-switched pulse drift with respect to the Q-switched firing time.
4. Conclusions

We have demonstrated a flashlamp-pumped Q-switched self-filtering unstable Nd:YAG single-frequency laser by using a phase-modulated ramp-fire technique. An RTP crystal is used for optical path length modulation. The single-frequency laser is capable of producing over 50 mJ pulse energy with a near-diffraction-limited spatial beam profile. The pulse duration is about 16 ns with a jitter of 183 ps, and the pulse-to-pulse energy stability is approximately 1%. The laser linewidth is measured to be less than 0.5 pm and the frequency stability is 14 fm over 30 min. The developed single frequency laser is scalable and harmonic, and is being incorporated into an OPO system.

Reference
[1] Kulatilaka W D Anderson T N Bougher T L Lucht R P 2005 Appl. Phys. 80 669
[2] Liu J L Liu Q Li H Li P Zhang K S 2011 Chin. Phys. B 20 312
[3] Kidd F G Narayanaswamy V Danehy P M Inman J A Bathel B F Cabell K F Hass N E Capriotti D P Drozda T G Johansen C T 2014 30th AIAA Aerodynamic Measurement Technology and Ground Testing Conference Jun 16–20, 2014 Atlanta, United States AIAA 2652
[4] Zhang G X Zhao Y F Zhang Y C Zhao P T 2008 Acta Phys. Sin. 57 7390
[5] Koechner W 2006 Solid-state laser engineering 6 New York Springer 268 271
[6] Schmitt R L Rahn R A 1986 Appl. Opt. 25 629
[7] Henderson S W Yuen E H Fry E S 1986 Opt. Lett. 11 715
[8] Fry E S Hu Q Li X 1991 Appl. Opt. 30 1015
[9] Zhang J Zhu X Zang H Ma X Yin S Li S Chen W 2014 Appl. Opt. 40 3046
[10] Bollanti S Lazzaro P Murra D Zheng C E 2002 Opt. Comm. 209 383
[11] Gobbi P G Morosi S Reali G C Zarkasi A S 1985 Appl. Opt. 24 26
[12] Roth M Tseitlin M Angert N 2005 Glass Phys. Chem. 31 86
[13] Chen S Lin W Shi F Huang J Li J Zheng H Lin J Xu C 2007 Chin. Opt. Lett. 5 223