Coherent Raman Scattering (CRS) microscopy, with contrast from coherent anti-Stokes Raman scattering (CARS) [1
] or stimulated Raman scattering (SRS) [2
], allows label-free imaging of biological samples with endogenous image contrast based on vibrational spectroscopy. Imaging with high spatial resolution and speed up to video-rate were demonstrated previously [3
]. Recently, SRS has superseded CARS as a contrast mechanism for microscopy, because it has improved sensitivity, no image artifacts from spectral distortions or coherent signal addition, and a linear concentration dependence. There has also been increasing interest in using CRS microscopy for medical diagnostics [4
], as well as practical applications outside the field of biological and medical applications, such as studies of two-photon polymerization reaction and mechanical properties of polymer microstructures [5
An important technical challenge in CRS microscopy is the requirement of two synchronized picosecond excitation sources. For practical CRS applications, the light source must fulfill the following requirements: (1) It must provide for two-color excitation, with at least one color tunable so that the difference frequency matches a molecular vibrational frequency. (2) The two colors must be temporally overlapped, with relative timing jitter much less than the pulse widths. (3) The spectral bandwidth of each color must be narrower than the bandwidth of Raman-active vibrational resonances. A transform-limited pulse width of several picoseconds is typically ideal. (4) Sufficient and comparable average power must be available in both beams.
Current excitation sources for CRS include two mode-locked lasers with a phase-locked-loop (PLL) and fine cavity adjustments to synchronize the pulses, optical parametric oscillators (OPOs) synchronously pumped by mode-locked lasers, and two-color sources based on fiber lasers and continuum generation [6
] or subsequent soliton self-frequency shift (SSFS) [7
]. Synchronized mode-locked lasers and OPOs are bulky, costly, and environmentally sensitive, while previous fiber-based sources either have limited spectral resolution due to the broad spectral bandwidth of femtosecond pulse [6
] or suffer from low output power [8
Time-lenses were used in active pulse compression [9
], telecommunications [10
], and pulse shaping [11
]. A time-lens imposes a temporal quadratic phase modulation onto the incident light, analogous to a spatial lens imposing a spatial quadratic phase onto a wavefront in space. The phase modulation broadens the spectrum of the input light, and generates the necessary spectral bandwidth for a short pulse. In practice, the quadratic phase modulation is approximated by applying a sinusoidal drive voltage to an electro-optic (EO) phase modulator. With proper dispersion compensation, picosecond or even femtosecond pulses can be generated from a CW laser [12
]. While the repetition rate of a conventional mode-locked laser is constrained by the cavity length, the repetition rate of a time-lens source is entirely determined by the electrical drive signal. Thus, a time-lens source has the remarkable capability of synchronizing to any mode-locked laser.
In this paper, we demonstrate a new scheme for synchronizing two pulsed sources for CRS. We synchronize a time-lens source to a tunable Titanium:Sapphire (Ti:Sa) mode-locked laser. The 1.7-ps, 240-mW synchronized pulse train from the compact, all-fiber time-lens source is well suited for CRS microscopy. In addition, electronic radio frequency (RF) delay is used to adjust the relative time delay between the two pulse trains for temporal overlap. By eliminating mechanical optical delay lines, the time delay of the excitation pulses can be adjusted over a large range without any perturbation of the spatial alignment.