Phase-sensitive optical coherence tomography using an Vernier-tuned distributed Bragg reflector swept laser in the mouse middle ear

Abstract

Phase-sensitive optical coherence tomography (PhOCT) offers exquisite sensitivity to mechanical vibration in biological tissues. There is growing interest in using PhOCT for imaging the nanometer scale vibrations of the ear in animal models of hearing disorders. Swept-source-based systems offer fast acquisition speeds, suppression of common mode noise via balanced detection, and good signal roll-off. However, achieving high phase stability is difficult due to nonlinear laser sweeps and trigger jitter in a typical swept laser source. Here, we report on the initial application of a Vernier-tuned distributed Bragg reflector (VT-DBR) swept laser as the source for a fiber-based PhOCT system. The VT-DBR swept laser is electronically tuned and precisely controls sweeps without mechanical movement, resulting in highly linear sweeps with high wavelength stability and repeatability. We experimentally measured a phase sensitivity of 0.4 pm standard deviation, within a factor of less than 2 of the computed shot-noise limit. We further demonstrated the system by making ex vivo measurements of the vibrations of the mouse middle ear structures.

Fig. 1

Schematic of phase-sensitive OCT: SS, swept source; C (A:B), fiber coupler with A to B coupling ratio; AT, attenuator; OD, optical delay line; PC, polarization controller; CIR, circulator; COL, collimator; SM, scanning mirror; OL, objective lens; BPD, balanced photodetector; FPGA, FPGA module; HOST, host computer; SPK; speaker. Blue lines: optical connections. Purple lines: electrical SMA connections. Black lines: electrical BNC connections.

Fig. 2

Phase sensitivity of VT-DBR swept laser. A-scans of a microscope coverslip autocorrelation (a) and a −40 dB reflector (c). The phase signal in the frequency domain from the peak signal [blue dot in (a) and (c)] based on a 10,000 line M-scan. The mean and standard deviations of theoretical phase noise are shown as solid and dashed red lines, respectively.

Fig. 3

Volumetric (a) and B-scan (b) OCT image of mouse middle ear. Anatomical structures (TM, tympanic membrane; M, malleus) are clearly displayed. The white dotted A-scan was selected for the sound-induced vibration measurement. Scale bar is 0.25 mm.

Fig. 4

Vibration magnitudes of the manubrium in ex vivo mouse middle ear. The vibration magnitudes were acquired with 10,000 A-scans with respect to the different sinusoidal frequencies and intensities [(a) 50 dB SPL at 4 kHz; (b) 70 dB SPL at 4 kHz; (c) 50 dB SPL at 8 kHz; (d) 70 dB SPL at 8 kHz] as a function of frequency. The maximum vibration magnitudes were (a) ~1 nm; (b) ~7 nm; (c) ~6 nm; (d) and ~55 nm.

Fig. 5

Vibration magnitudes of ex vivo mouse middle ear as a function of (a) sound intensity and (b) stimulus frequency.