Fourier Domain Mode Locked Laser and Its Applications

Abstract

The sweep rate of conventional short-cavity lasers with an intracavity-swept filter is limited by the buildup time of laser signals from spontaneous emissions. The Fourier domain mode-locked (FDML) laser was proposed to overcome the limitations of buildup time by inserting a long fiber delay in the cavity to store the whole swept signal and has attracted much interest in both theoretical and experimental studies. In this review, the theoretical models to understand the dynamics of the FDML laser and the experimental techniques to realize high speed, wide sweep range, long coherence length, high output power and highly stable swept signals in FDML lasers will be discussed. We will then discuss the applications of FDML lasers in optical coherence tomography (OCT), fiber sensing, precision measurement, microwave generation and nonlinear microscopy.

Keywords: Fourier domain mode-locking; frequency/time discretization; instability; laser and optics system; swept laser.

Figure 1

Schematic of a typical FDML fiber laser. The three main elements are the gain, tunable bandpass filter and fiber delay.

Figure 2

The operation principle of an FDML fiber laser from the viewpoint of (a) a single wavelength and (b) a given time.

Figure 3

The (a) integrated spectra and (b) waveforms of the FDML fiber laser harmonically mode-locked in the third order. The scan frequency of FFP-TF is 129.125 kHz and relative detunes are ±5 Hz and ±15 Hz. Reprinted with permission from Ref. [92].

Figure 4

Schematic of an FDML laser with an optional buffer stage: AWG, arbitrary waveform generator; LCD, laser diode controller; SOA, semiconductor optical amplifier; ISO, optical isolator; PC, polarization controller; BFP-TF, Fabry–Pérot filter; OSA, optical spectrum analyzer; PD, photo diode. Adapted with permission from Ref. [42].

Figure 5

Schematic of a broadband FDML laser which consists of two SOAs of adjacent wavelength bands arranged in parallel. Isolators are used before and after each SOA to ensure unidirectional operation. The SOAs are then coupled through two WDMs to complete the ring-cavity. Other broadband components include an FFP-TF for wavelength tuning, a fiber coil to set the cavity resonance frequency and a fiber coupler for output coupling. Adapted with permission from Ref. [73].

Figure 6

Schematic of an FDML laser that reuses the SOA in the laser cavity. SOA, semiconductor optical amplifier; ISO, isolator; PC, polarization controller; FFP-TF, fiber Fabry–Pérot tunable filter; OC, optical coupler; CIR, circulator; FR1 and FR2, Faraday mirror. Adapted with permission from Ref. [82].

Figure 7

Relative frequency shift induced by dispersion after propagation in 4 km SMF. The blue, red and black solid curves represent the frequency shifts caused by D₂, D₃ and their combined effect, respectively.

Figure 8

Schematic of an FDML laser with chirped fiber Bragg grating. It comprises a 1310 nm semiconductor optical amplifier (SOA), a fiber polarization controller (PC), a fiber optics isolator (ISO), a fiber Fabry–Pérot filter (FFP) driven at 411 kHz, a mixed fiber spool and a custom made chirped fiber Bragg grating (CFBG). Adapted with permission from Ref. [14].

Figure 9

(a) Example of raw interference fringe pattern with microring comb filter captured by the BPD. (b) Resampled interference spectrum with self-clocking. (c) Axial resolution estimation with PSF calculated from the signal of (b). The measured PSFs of frequency comb swept laser with microring comb filter for OCT imaging range from (d) 0 to 104 mm, (e) 0 to 1.5 mm and (f) 51.7 to 53.2 mm, covering the 6 dB sensitivity roll-off length. (g) The zoom-in view from 102.6 to 104.1 mm, covering the 15 dB sensitivity roll-off length. Reprinted with permission from Ref. [80].

Figure 10

Schematic diagram of an FDML laser with the motorized free space beam path (FSBP). Adapted with permission from Ref. [85].

Figure 11

Schematic diagrams of the self-starting, self-regulating FDML laser. Adapted with permission from Ref. [109].

Figure 12

(a) Schematic diagram of an FDML laser with time domain modulation and (b) the principle of discretization of swept signal with identical comb lines in the frequency domain. The blue solid lines are the rate varying pulses in the time domain and the red solid lines are the corresponding uniform comb lines in the frequency domain. Reprinted with permission from Ref. [87].

Figure 13

The (a) spectrum (the inset is a zoom-in view from 193 to 194 THz) and (b) temporal waveform of the discrete FDML laser with 100 GHz FSR. (c) The central frequency of the comb lines versus the channel number with a linear fitting. Reprinted with permission from Ref. [87].

Figure 14

The principle of discrete frequency domain harmonic mode-locked laser with reconfigurable uniform comb lines. (a) Six independent pulse train groups with the forward and backward sweeps in three sub-periods, (b–d) are the three sub-periods with bi-directional sweeps, where the relative shift of the comb lines is shown. Reprinted with permission from Ref. [95].

Figure 15

(a,c,e) show the resampled interference fringes in the frequency domain and (b,d,f) show the calculated point spread functions (PSFs) of discrete FDML laser with 300, 100 and 50 GHz FSR. Reprinted with permission from Ref. [95].

Figure 16

The principle of swept-source OCT.

Figure 17

En-face view with sequential numbers indicating (1) an arteriole, (2) a venule, and (3) one of the smallest vessels, respectively. (b) Frames taken from the projected 4-D movie (visualization 1) to show time-varying blood flow dynamics. (c) OMAG signal variations (spline fitted) to show blood flow dynamics in the functional vessels that marked in (b) and sequentially numbered in (a). Reprinted with permission from Ref. [128].

Figure 18

Experimental setup for an FBG sensor array system based on an FDML wavelength-swept laser. Adapted with permission from Ref. [133].

Figure 19

LiDAR based on an FDML laser achieves inertia-free imaging in one dimension with a high number of pixels and flexible imaging parameters. Adapted with permission from Ref. [147].

Figure 20

Schematic of the experimental setup to measure the DMGD in a few-mode fiber. Adapted with permission from Ref. [149].

Figure 21

Schematic to show the operations of an OEO based on FDML oscillator. Adapted with permission from Ref. [152].

Figure 22

Schematic of the time-encoded Raman system. (a) The fibre based, wavelength-swept FDML probe laser25. FC, fibre coupler; FFP-TF, fibre Fabry- Pérot tuneable filter; ISO, optical isolator; SOA, semiconductor optical amplifier. (b) The homebuilt fibre-based pump laser is digitally synchronized to the FDML. EOM, electro-optic modulator; WDM, wavelength division multiplexer; YDFA, ytterbium-doped fibre amplifier. (c) The lasers are combined in the beam delivery unit and focused onto the sample. The SRG signal is detected after subtraction of the offset by a differential balanced photodetector (BPD). Adapted with permission from Ref. [165].