Optical spectroscopy offers multiple advantages over other identification techniques because it is highly sensitive to the biochemical nature of materials, can offer great spatial resolution when needed and can be made highly portable. This has made it the technique of choice for multiple applications ranging from remote sensing to disease diagnosis. For instance, needs in remote sensing in the atmosphere or in the battlefield have driven significant developments in devices and techniques using infrared and Raman spectroscopy [1, 2, 3]. Because of the limitations of spontaneous Raman spectroscopy and its low scattering cross-sections, the field has moved towards nonlinear spectroscopy techniques such as Coherent Anti-Stokes Raman Scattering (CARS) or Stimulated Raman Scattering (SRS) spectroscopy. These coherent techniques offer advantages over spontaneous Raman spectroscopy such as a much higher sensitivity to concentration partly due to the enhanced scattering cross sections.
Needs in Nonlinear Spectroscopy
Nonlinear spectroscopy uses two or more synchronized laser pulses at different wavelengths to measure a spectroscopic property of a material such as a Raman active vibrational mode of a molecule. Several systems already provide some key characteristics that are required for nonlinear spectroscopy, such as good irradiances (i.e., peak power) and good tunability. Experimental methods that have shown great success for CARS spectroscopy and recently, SRS spectroscopy, are based on the use of optical parametric oscillators [4,5] or amplifiers or synchronized Ti:Sapphire lasers . They have been very successful for imaging lipids, myelin and water [7, 8, 9], and have shown some success in spectroscopy. However, they suffer from relatively slow tunability because they rely either on temperature or orientation tuning of nonlinear crystals in OPOs (typically seconds at best) or slit tuning in Ti:Sapphire lasers. For these reasons, schemes based on continuum generation have been used to mix a broadband spectrum with a narrowband pump while using a spectrometer to resolve the vibrational spectra . This has made possible spectral imaging studies of simple systems but has yet to be used in thick tissue: it suffers from low collection efficiency because of the low numerical aperture of spectrometers coupled with the requirements for tight focussing for spectral resolution.
In optical coherence tomography (OCT), a method based on coherence-gating of a reflected signal, a quantum leap in sensitivity was obtained when devices went from time domain to spectral domain.
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The first demonstration of very high speed OCT systems was made possible by the development of wavelength swept laser systems optimized for this application . Although many factors influence the quality of the data, the use of highly efficient detectors with narrow band detection can increase the signal-to-noise ration by several orders of magnitude. Similarly, the use of wavelength-swept sources for spectroscopy has several advantages over the use of broadband sources, one of them being that the detection system can now do without a spectrometer and substitute it with a large-area efficient detector, since the data is intrinsically time-encoded. For tissue measurements, this is a very attractive proposition but up to now, wavelength-swept systems for nonlinear spectroscopy were very limited (low peak power or extremely narrow tunability).
Genia Photonics has designed a laser source combo specifically tailored for the needs of nonlinear spectroscopists. In particular, it meets the demanding needs of the biomedical optics community while offering great opportunities in other markets. The system consists of two integreated fiber lasers that are synchronized: a programmable, dispersion-tuned, actively mode-locked fiber laser (PL) and a fiber master oscillator power amplifier (MOPA) whose outputs are combined through a wavelength division multiplexer (WDM) coupler (fig. 1).
The PL and MOPA contain not only an optical sub-system but high-speed electronic circuitry delivering precise short picosecond pulses to drive an electro-optic modulator. The root of the synchronization lies within a novel low-jitter function generator (FG) circuit capable of generating multiple signals that trigger the pulse generators of each laser. The FG outputs can be dynamically tuned to provide all the required delays to ensure synchronization of the pulses all the way to the target. Moreover, the delay can be dithered so that only each alternate pulse in the pulse train remains synchronized to perform temporally sensitive pump-probe experiments while maintaining the average power at a constant level.
The rapid tunability of the laser is achieved through the electro-optic modulator which serves as the active mode-locking mechanism that tunes a grating-based optical cavity (fig. 2). This allows the wavelengths to be selected in an arbitrary manner. Rapid and arbitrary tuning are two of the critical characteristics for spectroscopy.
The programmable laser can be tuned arbitrarily to any wavelength in the range 1524.0 to 1608.6 nm with a precision of 0.01 nm and can change wavelengths at a rate of 50,000 wavelengths per second. It is based on a dispersion-tuned actively mode-locked fiber laser with chirped fiber Bragg gratings (FBG) as the dispersive elements. Chirping of the Bragg gratings allows continuous change in wavelength and the wide tuning range is provided by the presence of FBGs pairs in the PL (fig. 3). The programmable laser produces 10 nJ of energy per pulse at repetition rates ranging from 12.4 MHz (1524 nm) to 14.0 MHz (1608 nm) with pulse widths around 25 ps.
The synchronization is obtained by triggering the MOPA from the PL pulse output. A wavelength-dependent delay is added by the delay generator to compensate for any dispersion internal or external to the cavity (fig. 4). Careful calibration ensures that the pulses are synchronized at the sample for any wavelength, enabling the acquisition of nonlinear spectra over the PL tuning range regardless of the optical path (fig. 4). The average powers of the PL and MOPA are 50 and 100 mW respectively.
As a demonstration that the power of the laser system is sufficient for nonlinear spectroscopy, figure 5 shows a Sum Frequency Generation (SFG) signal from a BBO crystal. The two lasers are combined and focused into a 4 mm BBO crystal, appropriately oriented for SFG between 1080 nm and 1550 nm with a wide acceptance bandwidth, where they generate a sum frequency signal through a third-order nonlinear process. The signal is observable with the naked eye and observed throughout the tunable range of the laser, where the SFG signal decreases mostly limited by the PL power. The spectrum was obtained by sweeping the PL wavelength at 420 discrete points with an integration time at each wavelength of 20 µs.
The system is fully compatible with nonlinear spectroscopy with its 30-picosecond pulses. The high average power (100 mW), repetition rate (10 MHz) provides very high peak powers sufficient for sum frequency, difference frequency, coherent anti-Stokes Raman scattering, and Stimulated Raman Scattering. It is expected that this laser will provide the much needed wavelength-swept source for nonlinear coherent Raman spectroscopy in tissue for medical diagnosis.
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