Two-color Semiconductor Mode-locked Laser system for Multiphoton Imaging Applications

Abstract

We demonstrate an all-semiconductor mode-locked laser system consisting of two external cavity mode-locked lasers operating at wavelengths 834 nm and 974 nm which use semiconductor optical amplifiers as gain media. The two-color laser system emits picosecond pulses with average powers of 25 mW and 60 mW resulting in peak powers exceeding 100 W and 80 W respectively. Synchronized output pulse trains from the lasers with a repetition rate of 282 MHz exhibit a relative timing jitter of 7.3 ps. Fiber coupled output from the laser system delivers an ideal output beam with TEM₀₀ mode profile. Peak power densities >1 GW/cm² can be achieved by focusing the output beam to a smaller spot with 4 μm diameter, which is crucial for applications that requires excitation of optical nonlinearities.

I. Introduction

COMPACT ultrashort pulse laser sources that can generate high peak intensities are of pivotal importance for applications such as multiphoton imaging, STED nanoscopy, dual-comb ranging and LiDAR [1]–[4]. Ultrashort pulse lasers that use solid-state gain media such as Ti:sapphire can generate femtosecond pulses with high energies and are most widely used in applications such as two-photon imaging and microscopy which uses nonlinear optical effects [5]. However, they are expensive, bulky and require maintenance. Semiconductor lasers are of low maintenance and offer benefits such as compactness, high wall plug efficiency and access to a myriad of wavelengths which can be done by tailoring the semiconductor material composition.

However, short upper state lifetime of 1 ns and low energy storage limits their ability to generate ultrashort pulses with very high energies. Lasers with breathing mode architecture [6] and Xtreme chirped pulse amplification (X-CPA) [7] techniques have been demonstrated in the past overcoming these issues. The X-CPA technique uses chirped fiber Bragg grating (CFBG) to stretch optical pulses beyond upper state lifetime of the semiconductor material. Stretched pulses are then sent through amplification stages resulting in efficient energy extraction from high power amplifiers by avoiding detrimental nonlinearites like integrating Self-Phase modulation (SPM) and carrier heating which are prevalent in semiconductor gain media [8], [9].

Recent advances in semiconductor manufacturing and chip design have enabled high power amplifiers like tapered amplifiers (TPA) and SCOWA’s to emit multiple watts of average power while maintaining single mode and diffraction limited beam quality. Building laser systems utilizing these devices can make short pulsed semiconductor lasers a viable candidate for multi-color two-photon imaging and ranging applications. In this paper, we demonstrate an electrically synchronized two-color hybrid mode-locked laser (HML) system that is versatile and capable of inducing fundamental nonlinear optical phenomena which are crucial for several applications.

The two-color laser system is described as follows: First we will describe the cavity configuration and output characteristics of the HML laser at 834 nm, followed by the HML laser at 974 nm. Next, we will discuss the optical amplification and compression of the laser output, noting specifically the need to avoid detrimental nonlinearities that can occur during amplification process. Finally, we will discuss the performance of the synchronized two-color laser system and it’s potential for applications in two-photon imaging.

II. External cavity mode-locked lasers

A. 834 nm laser

The external cavity mode-locked laser (MLL) show in Fig. 1 uses a GaAs/AlGaAs ridge waveguide semiconductor optical amplifier (RWGSOA) as gain media and a multiple quantum well saturable absorber (MQWSA) to initiate passive mode-locking (PML). The RWGSOA is placed at a position L/3 (L: cavity length) inside the cavity, and the MQWSA is placed at the back mirror position which also acts as a HR mirror. A 30T:70R partially reflective mirror is used as an output coupler. The RWGSOA is mounted p-side down on a copper stud for efficient heat dissipation and the waveguide is oriented at an angle 6° with respect to the optic axis of the cavity to minimize back reflections. Both facets of the RWGSOA are anti-reflection coated with Si₃N₄ to reduce Fresnel loss.

Fig. 1.

Schematic of the 834 nm laser. L: cavity length, MQWSA: Multiple quantum well saturable absorber, RWGSOA: Ridge waveguide semiconductor optical amplifier, OC: output coupler

PML is achieved by forcing the laser to operate within the excitonic absorption band (Fig. 2b) of the MQWSA. It is optimal to operate the laser within the excitonic absorption band because of the lower threshold to bleach the MQWSA and initiate the mode-locking process. Fundamental HML is implemented by adding a 282 MHz sinusoidal RF signal to the gain current using a bias-tee. Fig. 2a shows the optical spectrum at the MLL, amplification and pulse compression stages. The laser emits 40 ps (Fig. 2c) down-chirped pulses with an average power of 2 mW which are then amplified and compressed to 2.7 ps (assuming sech² pulse shape) using a single mode fiber (SMF), which will be discussed in section-III.

Fig. 2.

(a) Optical spectrum of the laser system at different stages (b) Transmission spectrum of the MQWSA [inset: Image of the MQWSA], (c) Intensity autocorrelation of the MLL after amplification and compression stages

B. 974 nm laser

The external cavity MLL shown in Fig. 3 uses an AlGaAs inverse bowtie semiconductor optical amplifier (IBTSOA) as a gain element and a transmission type MQWSA as a passive mode-locking element instead of a reflective type MQWSA due to the availability. The 974 nm laser operates in a harmonically mode-locked colliding pulse configuration which is different from the 834 nm laser because of the MQWSA used. The IBTSOA is placed at a position 2L/6 (2L: cavity length) inside the cavity, and the transmission type MQWSA is placed at position L which is half the length of the cavity. A HR mirror is used as a back mirror and a 30T:70R partially reflective mirror is used as an output coupler. The IBTSOA is mounted p-side down on a copper stud for efficient heat dissipation and the waveguide is oriented at an angle 6 ° with respect to the optic axis of the cavity to minimize back reflections. Both IBTSOA facets are anti-reflection coated with Si₃N₄ to reduce Fresnel loss.

Fig. 3.

Schematic of the 974 nm laser. 2L: cavity length, MQWSA: Multiple quantum well saturable absorber, IBTSOA: Inverse bow-tie semiconductor optical amplifier, OC: output coupler, HR: high reflective mirror

The laser is first passively mode-locked and HML is implemented by adding a 282 MHz sinusoidal RF signal to the gain bias current using a bias-tee. The 282 MHz RF signal corresponds to the 2^nd harmonic of the cavity. Fig. 4a shows the optical spectrum at the MLL, amplification and pulse compression stages. A significant improvement in the OSNR after compression stage is achieved by using an adjustable slit in the fourier plane of the grating compressor. The laser emits 50 ps (Fig. 4c) up-chirped pulses with an average power of 7 mW which are then amplified and compressed to 0.83 ps (assuming sech² pulse shape) using a 4f-dual grating compressor, which will be discussed in section-III.

Fig. 4.

(a) Optical spectrum of the laser system at different stages, (b) Transmission spectrum of the MQWSA [inset: Image of the MQWSA], (c) Intensity autocorrelation of the MLL after amplification and compresssion stages

III. Two-color laser system

The two-color laser system shown in Fig.5 consists of synchronized external cavity lasers, amplification and pulse compression stages. It should be noted here that optical amplification of the MLL pulse trains is performed by directly amplifying the chirped output pulses from each oscillator, followed by pulse compression to achieve final output pulse characteristics. This approach helps minimizing detrimental non-linearities that occur in semiconductor gain media such as dynamic carrier heating and free carrier absorption [8], [9].

Fig. 5.

Schematic showing the synchronized two-color mode-locked laser system with lasers centered at 974 nm and 834 nm, amplification, and compression stages. RWGSOA, IBTSOA: Semiconductor optical amplifier, MQWSA: saturable absorber, ISO: Isolator, λ/2: Half wave plate, TPA: Tapered amplifier, BT: Bias-Tee, RF syn: RF synthesizer, L: Aspheric lens, L1: Lens system consisting of two aspheric lenses and a cylindrical lens, SMF: single mode fiber.

The photodetected RF spectrum of the MLL’s shown in Fig. 6a looks identical to the RF source, emphasizing how the source dominates the RF characteristics of the MLL’s. The 974 nm MLL output is injected into a RWGSOA to boost the power from 2 mW to 10 mW. Isolators are placed between the MLL and amplifying stages to avoid any closed loop lasing. Output from the RGWSOA is injected into a 3mm long TPA which amplifies the signal from 8 mW to 100 mW with an applied bias current of 850 mA. Output from the TPA is coupled into an SMF with 50% coupling efficiency using a lens system that consists of an aspheric lens (f = 2.8 mm, 0.67 NA) that collimates light from the TPA, a cylindrical lens (f = 45 mm) to compensate for astigmatism and finally an aspheric lens (f = 4 mm, 0.6 NA). A 4f-dual grating compressor was used to compensate the chirp and compress the pulses from 50 ps to 0.83 ps (assuming sech² pulse shape) which is 4.2 times the transform limit (Fig. 4c). Considering all the insertion losses final output from the SMF was measured to be 25 mW which corresponds to a peak power over 100W.

Fig. 6.

(a) RF power spectrum of both MLL’s and the RF source, (b) sampling scope trace of the pulse train of the 834 nm laser

Output from the 834 nm laser is injected into a RWGSOA to boost the average power from 1 mW to 5 mW. The amplified signal is then injected into a 4 mm long TPA to boost the power from 4 mW to 130 mW with an applied bias current of 1600 mA. Output from the TPA is coupled into an SMF with 40% coupling efficiency using a lens system that consists of an aspheric lens (f = 3.1 mm, 0.7 NA) to collimate light from the TPA, a cylindrical lens (f = 45 mm) to compensate for astigmatism and finally an aspheric lens (f = 3.3 mm, 0.47 NA) to focus the light into the SMF. The SMF has a normal dispersion sign which is opposite to the sign of the chirp on the MLL pulses. So we chose to use the SMF to compress the pulses from 42 ps to 2.7 ps (assuming sech² pulse shape) which is 7.9 times the transform limit. This helps avoiding insertion losses introduced by a grating compressor.

We’ve intentionally limited the output power by operating the TPA’s at low gain current to avoid nonlinearities which causes red/blue peak enhancement resulting in pulse distortion [9]. This can be observed to a minor extent in the amplified spectrum of the 834 nm laser shown in Fig. 2a. However, these limitations can be overcome and higher powers can be achieved with negligible nonlinear effects, by implementing X-CPA technique in the current system [7]. Synchronization between the two lasers can either be achieved by driving both lasers with a common RF source or different sources. The repetition rate of the lasers can be de-tuned by Δf_rep without any moving parts by using different RF sources.

IV. Sum frequency generation and two-photon fluorescence

Cross-correlation measurements were performed to verify the synchronicity of the MLL’s for two cases: i) the lasers are driven by two different RF sources and ii) a common RF source driving the lasers. Fig. 2a shows the intensity cross-correlation measured by a commercially available SHG aurocorrelator, by combining the out beams from the lasers (Fig. 7). Relative timing jitter of 19.5 ps was observed when different RF sources were used to drive the lasers. A significant reduction in relative timing jitter to 7.3 ps (Fig. 8a) was observed when a common RF source (Agilent E4424B) was used along with careful optimization of the 974 nm laser cavity [10]. Timing jitter of the 974 nm laser inherently comes from the harmonic mode-locking technique used. By using a reflective type MQWSA and employing fundamental mode-locking similar to the 834 nm laser, relative timing jitter between the lasers could be further improved. Fig. 8b shows sum frequency generated by using a LiIO₃ crystal.

Fig. 7.

Schematic of the cross-correlation measurement and sum frequency generation setup that uses phase matching condition of LiIO₃ crystal.

Fig. 8.

(a) Measured cross-correlation of the lasers when lasers are driven with a common RF synthesizer and different RF synthesizers (b) Sum frequency spectrum generated from the synchronized lasers (c) Two-photon fluorescence in Pyrylium dye (left) and Rhodamine B (right) at the focused spot of the lasers

Phase matching is achieved by angle tuning the crystal. It should be noted that the optical spectrum of sum frequency (Fig. 8b) resembles a quasi-triangular shape, which is expected as the output from both 834 nm and 974 nm lasers have a flattop spectral density. We used pyrylium dye (50 μM) diluted in dichloromethane and Rhodamine B (10 μM) diluted in water to verify the feasibility of the lasers to induce two-photon excitation. Two-photon fluorescence was observed (Fig. 8c) when the laser was focused in to a cuvette containing the dye solutions. Considering a beam diameter of 21 μm at focused spot, estimated peak power density is over 40 MW/cm² at the focused spot.

V. Conclusion

We have demonstrated a two-color mode-locked laser system generating picosecond pulses centered at wavelengths 974 nm and 834 nm. Fiber coupled output from the laser system ensures ease of beam delivery and excellent beam quality with a TEM₀₀ mode. The laser system delivers an average power of 25 mW and 60 mW from the 974 nm and 834 nm lasers respectively. With estimated peak powers exceeding 100 W and 80 W, the laser system can generate peak power densities >1 GW/cm² when tightly focused to a smaller spot size of 4 μm. The demonstrated feasibility of the lasers in inducing two-photon fluorescence in different dye samples indicates that the laser system is a suitable candidate for inducing nonlinear optical phenomena in applications such as label-free Raman imaging, multiphoton imaging. Synchronicity between the lasers with a relative timing jitter of 7.3 ps was demonstrated using a common RF source. Repetition rate tunability of the lasers without any moving parts gives the system a unique feature that could be used in applications such as pump probe spectroscopy, time-resolved spectral analysis in mid-IR region and two-photon dual-comb lidar.