Highly functional wideband fiber lasers for metrology

Advances in ultrashort-pulse laser technology have generated several new applications in the fields of laser processing, biomedical optics, opto-electronics, and related fields.^{1, 2} However, conventional ultrashort-pulse lasers are limited by a requirement of quiet laboratory environments and water cooling. In addition, spectroscopy, optical coherence tomography (OCT), and nonlinear microscopy require both a wideband light source and a wavelength-tunable narrow-linewidth source.^{2, 3} Moreover, tuning range is generally limited by the laser's gain bandwidth. Although wideband wavelength-tunable ultrashort pulses can be generated, doing so requires a wavelength conversion technique that relies on nonlinear and mechanical fine tuning of the source crystals, which are cumbersome.

By contrast, fiber lasers are compact, stable, and practical.¹ Consisting of optical fiber devices, they are robust and maintenance-free, do not need water cooling, and only require electricity for their generation. However, the fiber laser's oscillation wavelength is limited by the gain bandwidth of the fiber amplifier and it cannot be varied over a wide range. To address this issue, we investigated highly functional ultrashort-pulse fiber lasers and wavelength-tunable ultrashort soliton-pulse generation.⁴ Solitons pulse with a particular balance of nonlinear and dispersive effects so that the temporal and spectral shapes of the pulses are preserved over long propagation distances. Using the ultrashort-pulse fiber lasers and nonlinear fibers, we can generate a broadened spectrum that covers a wide range of wavelengths, which we call a supercontinuum.

We recently developed a rapidly wavelength-tunable narrow-linewidth pulse source using comb-like dispersion fibers (see Figure 1).⁵ These comb-profile fibers compress the soliton pulse spectrum, demonstrating quasi-dispersion-increasing fibers using alternative splicing with conventional single-mode and dispersion-shifted fibers. In this set-up, the soliton effect provides a large degree of spectral compression. The wavelength can be tuned, up to 10GHz modulation speed, using an electro-optical modulator.⁶

Figure 1. Optical spectra of wavelength-tunable narrow-linewidth pulses. We compressed the pulse spectra of ultrashort soliton pulses using the nonlinear effects of a comb-profile fiber. We can tune the wavelength shot-by-shot using an intensity modulator. A.u.: arbitrary units.

In OCT, where fiber lasers are preferable over traditional crystal sources, we require deep penetration depths. However, in the biological setting, obstacles to deep penetration are the loss of water and hemoglobin (for example, in blood samples) and optical scattering. As the optical properties depend on the samples, we determined that the ideal approach was to select the center wavelength of the light source to improve light penetration for a given sample. For example, we generated a Gaussian-like wideband supercontinuum at 800–1700nm (see Figure 2), coupling the soliton pulse into the nonlinear fibers.^{7, 8} We generated supercontinua centered at 1700nm and demonstrated ultrahigh-resolution (UHR)-OCT analysis at that wavelength for the first time. For the generation of the supercontinuum, we started from the erbium-doped ultrashort-pulse fiber laser and then generated the wavelength-shifted ultrashort soliton pulse at 1700nm by nonlinear effects in fibers.⁴

Figure 2. Optical spectra of generated supercontinua using fiber nonlinearity at 800–1700nm. Arb.: arbitrary.

Using porcine trachea as a sample, we generated 3D UHR-OCT images using the supercontinua (see Figure 3). The axial resolution of OCT is determined by the center wavelength and bandwidth of the supercontinuum. At 800nm, we could achieve the highest axial resolution and observe the precise structure of tracheal tissue. At 1700nm, since the scattering decreases as the wavelength increases, we achieved both high axial resolution and deep penetration depth. We observed the deeper penetration depth particularly in the sample with low water content.

Figure 3. 3D ultrahigh-resolution optical coherence tomography images of porcine trachea using supercontinuum at wavelengths of (left) 800nm and (right) 1700nm.

In conclusion, we used spectrum compression of soliton pulses to develop a widely and rapidly wavelength-tunable narrow-linewidth source. We also generated a wideband supercontinuum using soliton pulses at several wavelengths for OCT, and we investigated the wavelength dependence of OCT imaging. We based the development of these highly functional wideband laser sources primarily on fiber devices. Very recently, we demonstrated ultrafast absorption spectroscopy using a wideband, rapidly wavelength-tunable narrow-linewidth source.⁶ We also produced 3D, non-invasive, cross-sectional imaging of protein crystals for the first time using UHR-OCT.⁹ We expect that our newly developed light sources will lead to novel applications, particularly in the field of metrology.