Optical coherence tomography (OCT) is a high-resolution biomedical imaging technique that allows three-dimensional imaging of biological tissues by measuring back-scattered light^[1]. The advantages of OCT include high resolution, noninvasiveness, and absence of radiation^[2]. Swept source OCT (SS–OCT) is the latest-generation OCT imaging technology, and there have been powerful advances in imaging technology which enable fast imaging speed^[3,4]. Swept sources are a key technology for SS-OCT. The primary challenge in SS-OCT is that it requires a high speed, wide wavelength bandwidth, and narrow instantaneous linewidth swept laser source^[5,6].

Several authors have studied how to develop high-performance wavelength swept laser sources. In 2006, Huber et al. provided a novel approach for increasing the tuning speed of a swept laser source called the Fourier domain mode locking (FDML) technique^[7]. By reducing the dispersion in the fiber delay line of the FDML swept laser source to eliminate intensity noise, the dispersion compensated setup was presented by several groups^[8–13]. In order to obtain increased sweep ranges and improve axial resolutions, Marschall et al.^[14] used two different amplifiers with complementary characteristics as a gain medium in the resonator. In addition, the combination of two semiconductor optical amplifiers (SOAs) in parallel has been successfully utilized in the swept laser source^[15–18], and two SOAs have substantially overlapped in gain spectra. In addition, good roll-off has been demonstrated by several groups^[19–21].

However, the previously reported high-performance FDML swept laser sources are all based on the improvement of the performance of optical devices, and there are few articles on improving the performance of device driving. There are also insights inside the swept laser source which can develop a high-speed, wide bandwidth FDML by improving the performance of optical device driving and also reduce significant cavity loss, cost, and complexity.

In this paper, we describe the development of a new technique ranging phase controllability as a tool for making a novel driving modulation method of a fiber Fabry–Perot tunable filter (FFP-TF) and demonstrate it for the wide bandwidth of an accessible FDML swept laser. The primary insight that enabled this result is to avoid the discontinuous part of the spectrum by changing the trend of driving voltage value intermittently. A detailed structure framework of phase controllability driving is provided, and the swept laser source achieves broader bandwidth through modulating the driving phase of FFP-TF within the cavity. To the best of our knowledge, the achieved sweep rate of 100.5 kHz and full width at half-maximum (FWHM) bandwidth of 152.25 nm at a center wavelength of 1335.45 nm to the widest bandwidth among swept laser sources using a single SOA at 100 kHz are reported thus far in a 1.3 µm wavelength range. This advance enables high-performance SS-OCT imaging without extra-cavity dispersion compensation, significantly reducing swept source cost and complexity. Our techniques are general and can be applied to swept laser sources and SS-OCT, as well as fiber-optic sensing or interferometric applications.

A key component in FFP-TF is piezoelectric ceramics, which will produce different deformations caused by different DC voltages. Due to different deformations of piezoelectric ceramics, the distance between two high-reflection mirrors of FFP-TF is changed, thus filtering out different wavelengths. Figure 1 shows the relationship between deformation displacement and DC voltage of piezoelectric ceramics (AHB700C801). The deformation curve of piezoelectric ceramics is non-overlapping when the voltage increases or decreases and nonlinear. Although the same DC voltage value is used for driving, the deformation displacement of piezoelectric ceramics is different with different DC voltage changing trends (the DC voltage value increases or decreases).

Figure 2 shows the swept spectrum with respect to different DC voltage values. The driving DC voltage increases from 2 V to 6 V [as shown in Fig. 2(a)], then it decreases from 6 V to 2 V [as shown in Fig. 2(b)], and the swept spectrum is recorded every 1 V. In the same way, the driving DC voltage of FFP-TF is increased from −6V to −2V [as shown in Fig. 2(c)] and then reduced from −2V to −6V [as shown in Fig. 2(d)]. Tables 1 and 2 record the values of the central wavelength under different driving voltages. The center wavelength of the swept output spectra is not constant and changes irregularly even when the driving voltage is the same. The discrete center wavelength of the sweep output is not continuous, and the center wavelength by the values at both ends of the driving voltage range is not within the overall filtering wavelength curve. The same rule applies when the driving voltage is negative.

Periodic sinusoidal signal modulation of the FFP-TF has also been utilized for the driving mode of swept laser sources^[22–24]. However, through the above analysis of piezoelectric ceramics inside the FFP-TF and the change of the center wavelength of the spectra, we know that the driving of the sinusoidal signal can work normally, but it has some limitations. The key technological challenge is to change the driving trend in the wavelength gap to provide an extended and continuous output spectrum. In order to solve this problem, we propose to change the real-time voltage of the driving of FFP-TF by tuning the phase of the sinusoidal signal. Figure 3 is a schematic showing the concept of the output optical bandwidth in a wavelength swept laser source under the driving modulation method with a standard sinusoidal signal [as shown in Fig. 3(a)] and a phase adjustable sinusoidal signal [as shown in Fig. 3(b)]. With a standard sinusoidal signal [Fig. 3(a)], the rising edge and falling edge are symmetrical. However, due to the fact that the change of central wavelength is not continuous when the FFP-TF is driven by a standard sinusoidal signal, the output optical bandwidth is not fully widened. It is clear that the central wavelength varying trend is consistent with the driving modulation of FFP-TF. The key technological challenges and solutions are changing the driving trend of the segmented wavelength, so the segmented wavelength is spliced with the previous wavelength, and the output spectrum changes continuously and broadens. To match the resolutions, we developed a driving modulation method, which we named ‘phase adjustable sinusoidal signal’ [Fig. 3(b)]. The voltage driving FFP-TF is changed by changing the phase of the sinusoidal signal and broadening the output spectrum. The waveform of the driving of FFP-TF in a single cycle is changed by changing the phase of the sinusoidal signal and broadening the output spectrum. A detailed description of the already driving modulation method designs and results from the feasibility analysis studies performed above is now given in the following.

Figure 4 shows a schematic of the block diagram of driving of the phase adjustable sinusoidal signal [as shown in Fig. 4(a)] and the FDML laser resonator [as shown in Fig. 4(b)]. The reference clock (fc) provides the initial frequency to the frequency controller module. Phase tuning is carried out by pressing the key control module, and the results of the accumulator module are processed by the adder module. The waveform controller module controls the frequency and amplitude of the output signal. The digital to analog converter and low-pass filter modules work together to output phase adjustable low-noise analog signals. Figure 4(b) is a schematic of the specific configuration used in this manuscript. The laser oscillator comprised an SOA as broadband gain medium centered at 1320 nm, and the intracavity SOA current and temperature were modulated using a home-built high-precision temperature control driver. An FFP-TF as the tunable, narrowband optical bandpass filter was used. The FFP-TF is driven periodically using a phase adjustable signal generator, and the driving frequency of the tunable wavelength filter is determined by the cavity length. The output sweep signal is detected using a spectral analyzer (from Yokogawa, Inc., AQ6374) to verify the output spectrum of the frequency swept laser source, and a 45 GHz IR photodetector (from New Focus, Inc., Model 1014) and a digital oscilloscope (from Tektronix, Inc., SDA-820Zi-B) at 80 GS/s with 8 bit resolution are used to verify transient intensity profiles.

Figure 5 shows the laser output spectra and its transient intensity profiles of the wavelength swept laser sources for using a phase adjustable sinusoidal signal driving waveform. It is important to note that in addition to the sinusoidal signal parameters (frequency, voltage amplitude, voltage offset, and phase), the phase of sinusoidal signal is also necessary in order to generate a broader bandwidth. Output spectra of wavelength swept laser sources are shown in Fig. 5(a). The edge-to-edge tuning range is 152.25 nm from 1259.33 to 1411.58 nm. Figure 5(b) shows transient intensity profiles of the spectrum shown in Fig. 5(a) at a sweeping rate of 100.5 kHz. The transient intensity profiles at both sweep ranges were significantly higher than those within the center because derivatives at the peak and trough of the driving signal were zero, and the deformation of the FFP-TF changed to zero rapidly, while the transmission peak passed through the two ends of the swept range.

To characterize the imaging depth of the FDML swept laser source, we tested the performance of the FDML source for OCT imaging in the setup. Figure 6 shows sensitivity roll-off as a function of optical path difference. Sensitivity is measured by placing a partial reflector in the sample arm. The maximum sensitivity of the FDML swept laser source is 40 dB at a depth of 0.25 mm, as shown in Fig. 6. A decrease of approximately 3 dB within a depth range of 2.045 mm is measured.

Figure 7 illustrates the details of one of the point spread functions (PSFs) showing an imaging depth of 0.25 mm for phase adjustable sinusoidal signal operating modes. The measured FWHM resolution of the PSF of the fast Fourier transform (FFT) amplitude fitted by a Gaussian function was 6.5 µm in air. It corresponds to the effective axial resolution of ∼4.64µm in tissue (n=1.4).

In conclusion, we presented a new driving modulation method of FFP-TF, the ranging phase controllability. This technique has numerous advantages over previously described bandwidth widening techniques. First, it is very power efficient, the driving voltage of the FFP-TF is changed by adjusting the phase of the driving signal, and the change of bandwidth can be seen in real time. In addition, this approach improves the effective swept bandwidth through optimizing the driving circuit of FFP-TF without the need for extra-cavity dispersion compensation, significantly reducing system cost and complexity. Our work also provides new general insights into advanced problems associated with high-performance swept source lasers. All of our techniques are general, and we firmly believe that this work would be beneficial to the OCT society, but also more generally to fiber-optic sensing or interferometric applications.