High-power, dual-wavelength femtosecond LiB3O5 optical ... - NSFC

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providing a 520 nm pump laser with durations of 250 fs at a repetition rate of 57 MHz. High efficiency .... with temperature changes, lower oscillation thresholds.
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OPTICS LETTERS / Vol. 39, No. 13 / July 1, 2014

High-power, dual-wavelength femtosecond LiB3O5 optical parametric oscillator pumped by fiber laser Chenglin Gu, Minglie Hu,* Jintao Fan, Youjian Song, Bowen Liu, and Chingyue Wang Ultrafast Laser Laboratory, Key Laboratory of Optoelectronic Information Science and Technology, Ministry of Education, College of Precision Instruments and Optoelectronics Engineering, Tianjin University, 300072 Tianjin, China *Corresponding author: [email protected] Received March 21, 2014; revised May 18, 2014; accepted May 22, 2014; posted May 30, 2014 (Doc. ID 208643); published June 24, 2014 We reported a dual-wavelength femtosecond optical parametric oscillator (OPO) based on a temperature-tuned LiB3 O5 crystal. The OPO was synchronously pumped by a frequency-doubled, mode-locked Yb-fiber laser amplifier, providing a 520 nm pump laser with durations of 250 fs at a repetition rate of 57 MHz. High efficiency and dualwavelength operation are obtained over the ranges of 658–846 nm and 2.45–1.35 μm. The observed dual-wavelength tuning is in agreement with the values predicted by numerical simulation. Moreover, a sum-frequency yellow laser of the longer signal and idler tunable from 555 to 623 nm with practical power is achieved. With an 8% output coupler, the maximum signal output power is 390 mW pumped at 3 W with dual-wavelength operation, while the maximum power of sum-frequency generation is 110 mW at 590 nm. © 2014 Optical Society of America OCIS codes: (140.3600) Lasers, tunable; (140.7090) Ultrafast lasers; (320.7090) Ultrafast lasers; (320.7110) Ultrafast nonlinear optics. http://dx.doi.org/10.1364/OL.39.003896

Optical parametric oscillators (OPOs) with widely tunable radiation from UV to mid-IR are now practical sources in labs, and they have been widely deployed in a number of applications, including biophotonics, optical microscopy, and time-resolved spectroscopy. Among them, OPOs which operate at dual wavelengths have achieved additional attention for potential advantages in multiwavelength optical microscopy [1,2], frequency metrology, coherent pulse synthesis [3,4], and terahertz [5] or midinfrared generation [6] using nonlinear downconversion. To realize the dual-wavelength operation, two oscillated wavelengths should be phasematched simultaneously. A widely used method is to use two different crystals in a single OPO—for example, crystals cut at two different angles or controlled in two different temperatures. Another common method is to use a single periodically poled quasi-phase-match crystal with dual gratings [7]. For ultrafast-pulse-pumped OPOs, an OPO with a single nonlinear crystal can operate at dual wavelengths only when two pulses with different wavelengths have the same cavity group velocity and are phase-matched simultaneously [8]. Lithium triborate (LBO) crystal with excellent optical and mechanical characteristics combined with the availability of high-power laser sources has led to a revival of interest in OPOs as practical tunable light sources. LBO crystals can be operated at or near room temperature with critical phase matching and temperature tunable. Dual-wavelength oscillation could be sustained in the LBO OPO when pumped with a green laser due to its dual-phase-matching characteristic [9]. With picosecond [10] and nanosecond laser pumping [11], the dual-wavelength operations have been demonstrated in LBO OPOs where taking group velocity into consideration is unnecessary. In this Letter, we report a high-power tunable dual-wavelength femtosecond LBO OPO pumped by the second harmonics of a femtosecond fiber laser. Independent tuning of the dual-signal wavelength is achieved by a double-cavity configuration, 0146-9592/14/133896-04$15.00/0

which can be conveniently changed to single-wavelength operation. With an 8% output coupler (OC), the maximum signal output power is 390 mW pumped at 3 W with dual-wavelength operation. The dual wavelength of the signal can be continuously tuned from (680, 730) nm to (651, 846) nm. Additionally, sum-frequency-generation tuning from 555 to 623 nm of the longer signal and idler is obtained, with the maximum power of 110 mW at 590 nm. The configuration of the femtosecond LBO OPO is shown in Fig. 1. The OPO was pumped by the frequency-doubled pulses of a diode-pumped, mode-locked Yb-fiber laser-amplifier system whose repetition rate and center wavelength are 57 MHz and 1040 nm, respectively. A similar amplifier system is demonstrated in [12]. The oscillator is using nonlinear polarization evolution (NPE) as a passive mode-locking (PML) mechanism. The seed laser emits an average power of 21 mW at 1040 nm. The amplifier is a 2.5 m Yb-doped, large-mode-area crystalfiber-based system. The amplifier can output 8.5 W average power when it is pumped by a 25 W laser diode centered at 976 nm. After compression by a pair of transmission gratings (TGs) (1250 lines∕mm, Ibsen photonics), pulses as short as 180 fs can be obtained with

Fig. 1. Experimental setup of dual-wavelength LBO OPO. M1–M4: mirrors, OC: output coupler, L1 and L2: lens, HWP: halfwave plate, PBS: polarizing beam splitter, P1 and P2: CaF2 prism pair. © 2014 Optical Society of America

July 1, 2014 / Vol. 39, No. 13 / OPTICS LETTERS

as high as 6.2 W average power left. A half-wave plate combined with a polarization beam splitter is used to control the output power of the femtosecond laseramplifier system. L1 (f  100 mm) and L2 (f  90 mm) are the focusing and collimating lenses for second harmonics generation (SHG), respectively. A 2 mm thick LBO is placed in between as the SHG crystal whose cutting is φ  90°, θ  12.9° for type-I (e → oo) interaction. The maximum SHG power is 3 W, corresponding to 48.4% conversion of the fundamental laser. The pulse duration at 520 nm could be 250 fs, with calculation considering the temporal walkoff between fundamental- and secondharmonic pulses. A green pump laser (520 nm, SHG) is focused by a convex lens L3 (f  80 mm) and has a Gaussian diameter around 80 μm at the position of the LBO crystal. M1 to M4 are cavity mirrors with 99.8% reflectance efficiency from 550 to 900 nm. They are highly transparent (T > 95%) for the green pump laser. Concave mirrors M1 and M2 with r  150 mm provide the focusing for the OPO crystal. M3 and M4 are plane mirrors mounted on the delay line to adjust the OPO cavity lengths. The output of the OPO is delivered through an 8% OC to signal diagnostics. P1 and P2 combine a CaF2 prism pair. The OPO crystal is a 4 mm long noncritical xy-plane phasematched LBO (φ  90°, θ  0°) housed in an oven which can be adjusted from room temperature to 200°C for type-I (e → oo) interaction. LBO was selected as the nonlinear medium for its high optical damage tolerance, broadband transparency, and temperature-tuned noncritical phase-matching capability. This phase-matching configuration has the advantage of broad tuning potential with temperature changes, lower oscillation thresholds owing to the absence of beam walkoff, and a highly effective nonlinear coefficient (deff  d32 ∼ 1.2 pm∕V) [13]. The OPO is protected in an enclosure to avoid ambient disturbance. Hours of operation were achieved, but there were still several-nm wavelength drifts. To further increase the stability of the OPO, a negative-feedback stabilization mechanism of the cavity length should be introduced. The dual-wavelength operation of an OPO will be obtained as long as the two wavelengths have identical cavity optical length due to the dual-phase-matching property of the LBO crystal, but the group velocities mismatch (GVM) brought by the nonlinear crystal and the broadband dielectric mirrors should be compensated for in the femtosecond-laser case. Wedge pairs and chirp mirrors are commonly used, but complex adjustments like mirror changing are inevitable when the tuning range of the wavelength is as large as in OPOs. Prism pairs can also be used in dual-wavelength femtosecond lasers to separate the spectrum [14]. Here, we present a new type of OPO cavity for dual-wavelength operation with a prism pair. A prism pair is introduced to separate the different wavelengths in the spectrum plane, and independent delay lines ended by M3 and M4 are set up for different wavelengths. The distance between the prisms in the experiment is around 40 cm, and the signal wavelengths shorter than 680 nm and longer than 730 nm are available and well separated. The signal gap from 680 to 730 nm is caused by the gap between M3 and M4, and we believe that this gap can be reduced by further increasing the

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Fig. 2. Evolution of the dual-wavelength spectra of the LBO OPO at a temperature of 140°C.

prism-pair distance. By carefully adjusting both the delay lines, two signal wavelengths can oscillate simultaneously. Figure 2 shows the tuning property of dualwavelength operation at a temperature of 140°C. We tuned the longer signal wavelength by changing the cavity length, moving M4 while keeping the shorter signal wavelength fixed around 670 nm. The longer signal can be tuned continuously from 730 to 775 nm. As shown in Fig. 2, the longer signal wavelength was tuned from 735 to 773 nm with the output power varied from 270 to 300 mW and 3 W green laser pumping. This is due to the broad-signal phase-matching bandwidth when pumped by wide spectrum femtosecond pulses. During dual-wavelength operation, sum-frequency generation was obtained. Figure 3 shows the spectra of the corresponding sum-frequency yellow laser. The yellow laser was generated by the phase-matching SFG of the longer wavelength signal and the longer wavelength idler in the LBO. It can be tuned from 555 to 580 nm. Keeping the LBO temperature at 140°C, the maximum output power of the SFG is 75 mW at 555 nm (the reflectance of P1 was unconsidered).

Fig. 3. Corresponding sum-frequency-generation spectra in dual-wavelength operations of the LBO OPO at a temperature of 140°C.

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Fig. 4. (a) Calculated signal, idler, and sum-frequency wavelength of 520 nm pumped LBO OPO. (b) Calculated signal and idler wavelength of 520 nm pumped LBO OPO (black solid curve) and corresponding experimental signal wavelength (red squares). For T  140°C–170°C, additional sum frequency is generated in the LBO crystal (555–623 nm) (red circles). (c) Wavelength-tuning property with the temperature of the OPO. The left area of the red dashed line is the dual-wavelength region, and each color indicates the two signal spectra; the right area of the red dashed line is the single-wavelength region above 170°C.

The temperature-tuning property of the LBO OPO is characterized from 140°C to 200°C. For ultrafast OPO, the phase-matching bandwidth of the signal is relatively larger due to the broadband spectrum of the pump. Figure 4 shows the temperature-tuning property of the LBO OPO. The green pump laser is centered at 520 nm with a 6 nm bandwidth. Figure 4(a) shows the calculated phase-matching wavelength of the LBO OPO. The upper curve is the typical dual-wavelength phase-matching property of the LBO OPO pumped at 520 nm. The degeneracy of the signal is around 710 nm, while the corresponding idler is 1940 nm. The lower curve indicates the additional phase-matching SFG of the signals and idlers in the LBO. Figure 4(b) shows the corresponding experimental results. The 20°C right shift between Figs. 4(a) and 4(b) is caused by temperature differences between the crystal and the oven, because all the temperature values we measured are from the oven itself, while the temperature of the crystal is relatively lower. Because of the dispersion property of the LBO crystal, the bandwidth of the upper signal is larger than that of the lower signal. For example, the bandwidth of the upper signal is about 50 nm at 825 nm, compared to 10 nm at 660 nm of the lower signal. The red squares indicate the measured signal bandwidths according to temperature. The upper limitation of the signal is 913 nm, which is caused by the mirror coating, shown by the dashed line in Fig. 4(b). When the OPO is under dualwavelength operation, additional SFG from 555 to

623 nm is observed between 140°C and 165°C, shown by the red circles in Fig. 4(b). The maximum SFG output power is 110 mW at 590 nm when the LBO temperature is set to 150°C. Figure 4(c) shows the corresponding spectra tuning with the temperature. The red dashed line separates the dual-wavelength operation from singlewavelength operation. When the OPO is pumped by a 3 W green laser, the optimized signal’s output power (8% OC) according to the temperature setting is shown in Fig. 5. The red squares indicate the dual-wavelength oscillating region, and the black circles show the single-wavelength operation. The maximum signal output power, 390 mW, could be observed at 145°C with dual-wavelength (670, 750) nm operation. Meanwhile, 72 mW SFG at 566 nm could also be measured. Hence, the total corresponding conversion efficiency is 15.4%. The black triangles indicate the average power of yellow SFG under different temperatures. The maximum power of the SFG is 110 mW at 590 nm when the crystal is heated to 150°C. A typical intensity autocorrelation (dual-wavelength operation) profile of the signal is shown in the inset of Fig. 5. With a Gaussian pulse shape assumption, the pulse duration is 230 fs, close to that of the pump laser. Comparing with the calculated transform-limited duration, 200 fs, there is only a little chirp of the signal. A typical signal output power in terms of the pump power is shown in Fig. 6. Dual-wavelength operation at 666 and 787 nm is achieved when the temperature is set to 150°C and the corresponding idler wavelengths are 2372 and 1532 nm, respectively. The 590 nm phasematched SFG of the longer signal 787 nm and the longer idler 2372 nm is obtained with practical power simultaneously. The threshold of the OPO is 597 mW, and by increasing the pump laser to 3 W, the signal power reaches 380 mW. At this point, the corresponding conversion efficiency is 12.7%, and the slope efficiency of the system is 15.8%. In summary, we have reported a femtosecond noncritical LBO OPO pumped by the SHG of a Yb-doped fiber laser. A prism pair has been used to achieve dual-wavelength OPO operation, and free tuning is obtained by

Fig. 5. Signal output power dependent on temperature in the dual-wavelength oscillated region (red squares) and the singlewavelength region (black circles); the black triangles indicate the yellow SFG laser power; the inset shows the typical autocorrelation of the signal.

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This research is supported by the National Basic Research Program of China (Grants Nos. 2011CB808101 and 2010CB327604), the National Natural Science Foundation of China (Grants Nos. 61322502, 61205131, and 11274239), and the Open Fund of the State Key Laboratory of High Field Laser Physics (Shanghai Institute of Optics and Fine Mechanics).

Fig. 6. Signal and SFG power as a function of the pump power at a temperature of 150°C. The OPO is operating at dual wavelengths of 666 and 787 nm, and the SFG is at 590 nm.

independently tuning the two branches of signal cavity lengths. When setting the temperature of the crystal between 140°C and 170°C, the LBO OPO generates not only simultaneously two pairs of parametric waves but also the SFG (555–623 nm) of the longer signal and idler with practical output power. The femtosecond parametric pulses with five different colors can overlap in time and spatial domain if one is using properly coated mirrors. Such an approach draws significant attention in many applications, such as multiwavelength optical microscopy, frequency metrology, and so on. The presented experimental and theoretical results provide a further understanding of the physical properties of femtosecond LBO OPO. And this will certainly be profound for the optimization of OPOs in various applications.

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