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FDML laser

The home-built, temperature-stabilized FDML laser consists of a semiconductor optical amplifier (SOA, BOA 1132 S, Thorlabs), a fibre-based delay line cavity, mostly consisting of HI1060 fibre, and a home-built fibre based tuneable Fabry-Pérot optical bandpass filter (FP). A chirped fibre Bragg grating (CFBG) is used for dispersion compensation. An optical isolator (ISO) ensures unidirectional operation, and a polarization controller (PC) is used to optimize the polarization and hence the lasing and the laser power. The filter is periodically tuned synchronously to the optical roundtrip frequency of 411 kHz. The filter drive frequency is actively controlled by a closed feedback loop over a few 10^ths of mHz to compensate remaining drift caused by imperfect cavity temperature stabilization, which has a short time stability of ~0.01 °C (see Fig. 4 in Pfeiffer et al.²⁹). With an actively controlled sweep frequency the FDML laser can be kept in sweet spot operation mode^28,29. In sweet spot operation or ultra-stable regime FDML lasers hardly exhibit any excess intensity noise. In typical FDML lasers it can be achieved when the remaining dispersion of the ~500 m long fibre cavity is less than ~200 fs²⁹. In short, this is achieved with a chirped fibre Bragg grating, careful selection of the fibre types and a temperature gradient at the grating. The centre wavelength of the FDML laser is 1300 nm and the maximum achievable sweep bandwidth, limited by the gain width of the SOA, is 140 nm. For a longer beat signal the bandwidth was smaller: 40 nm for the presented data in Fig. 2, Fig. 3 and measurement 1 and 2 in Fig. 4 and 80 nm in measurement 3 and 4 in Fig. 4. The FDML laser output is modulated with a duty cycle of 40%. Usually, the laser is operated with a duty cycle of 12,5%, but it has been extended to obtain a longer beat signal.

CW laser

The two identical tuneable ring laser cavities each consist of a semiconductor optical amplifier (SOA, BOA1132, Thorlabs) and an optical isolator (ISO), connected by polarization-maintaining fibres. A free space beam path with tiltable transmission gratings (GTI25-03A, Thorlabs) serves as a tunable wavelength-selective element. A fibre coupler (FC) serves as output. The lasers are operated in continuous wave mode, the wavelength can be adjusted from ~1240 nm–1380 nm. By superimposing the light of the two CW lasers their stability and linewidth are monitored. The two lasers were always set to a wavelength difference of about 0.05 nm to generate a 10 GHz beat signal. The optical spectra are measured with an OSA and their interference signal is detected with a photodiode (Discovery Semiconductors DSC20H, 35 GHz, PD 2) and the real-time oscilloscope. If both lasers have no mode hops during measurement, the Fourier transform of this beat signal has a narrow tall peak and one laser can be used for the superposition with the FDML laser. The CW laser’s short-term linewidth is <10 kHz.

Measurement setup

The superposition of FDML and CW laser is detected with a fast photodiode from Finisar (XPDV2320R, 50 GHz 3 dB roll-off, PD 1). The data is recorded with a 63 GHz real-time oscilloscope (DSOZ634A Infiniium, Keysight). The raw data is freely available online³⁶. We note that a measurement with a radio frequency spectrum analyser would only record a time-averaged signal due to the fast sweep rate of up to >10¹⁹ Hz s⁻¹ of the FDML laser. Additionally, the high speed of the oscilloscope allows to fully characterize the light field of the FDML laser, as this frequency represents the upper limit of the laser output noise. This is due to the unique setup of the FDML principle, where the optical bandpass filter of the intra-cavity Fabry-Pérot-filter acts as a low-pass filter for the maximum frequency of phase and amplitude fluctuations. The filter with an optical bandwidth of ~0.165 nm corresponds to a frequency limiter of only ~29 GHz at 1300 nm. Therefore, any optical phase or amplitude fluctuations from the laser are slower than the electronic bandwidth of the detection. The OSA to measure the optical spectrum of the FDML laser is from Hewlett Packard (86142 A). The spectra of the CW lasers are detected with an OSA from Yokogawa (AQ6370).

The beat signal measurement is a superposition of the ideal harmonic wave of the ring laser

and a chirped signal

of the FDML laser. The sweep operation of the FDML laser follows closely the tuning operation of the intra-cavity filter. This is a Fabry-Pérot filter with sinusoidal tuning, so the wavelength is given by \(\lambda \left(t\right)={\lambda }_{0}+C\cdot {{\sin }}\left(2{{\uppi }}{\nu }_{{{{{{\rm{FDML}}}}}}}\cdot t\right)\) where \({\lambda }_{0}\) is the centre wavelength and \(C\) the half tuning range. Since we only analyse about 20 ns out of a 2.4 µs filter tuning cycle, we can linearize the instantaneous FDML frequency to \({\nu }_{{{{{{\rm{FDML}}}}}}}(t)={\nu }_{0}+\alpha \cdot t\) where α is a constant in Hz s^-1 indicating the change in optical frequency of the FDML laser per second. \({\nu }_{0}\) is the starting frequency of the FDML laser, \({A}_{{{{{{\rm{CW}}}}}}}\) and \({A}_{{{{{{\rm{FDML}}}}}}}\) the amplitudes of CW and FDML laser, \({\nu }_{{{{{{\rm{CW}}}}}}}\) the frequency of the CW laser and \({\varphi }_{{{{{{\rm{CW}}}}}}}\) and \({\varphi }_{{{{{{\rm{FDML}}}}}}}\) are absolute phase offsets of the CW and FDML laser.

Swept phase continuity

The FDML laser is modulated with a duty cycle of 40% (rainbow in Fig. 2a). Within a sweep the wavelength changes from 1310 nm to 1270 nm (backward sweep). The wavelength of the CW laser is 1305 nm. Beat signal traces were measured for 200 µs or 83 successive sweeps. The beat signals were tenfold upsampled through zero padding in the frequency domain for better visibility of the smooth swept phase continuity especially at the edges of the beat signals. It was verified that the upsampling did not change the results of the following data analysis methods.

Single-mode phase evolution

The trace is cut every 2.4 µs, corresponding to the reciprocal of the FDML repetition rate of 411 kHz. Successive sweeps were overlaid, and the amplitude was compared at one time-point (Fig. 3a, b) generating a periodic phase evolution (Fig. 3c). An FFT is applied to this single-mode phase evolution. The revealed spectrum is a convolution of the spectrum of the FDML laser and the spectrum of the CW laser. Since the single-mode phase evolution is made of one data point per beat signal, the sampling rate corresponds to the FDML frequency of 411 kHz. Therefore, the Nyquist frequency is 205.5 kHz and the frequency resolution is 4.95 kHz. The linewidth of the superposition of the two CW lasers was 9 kHz. Since this is also a convolution of the spectra of the two single CW lasers, it is only an upper limit for the linewidth of one of the CW lasers. The presented instantaneous linewidth of 12 kHz is the specific value for the presented dataset at the presented spectral position. We measured somewhat different values in the range of 5 kHz to 25 kHz depending on the spectral position and the dataset.

The shown spectra of the single-mode phase evolution are amplitude spectra, which are used to determine the instantaneous linewidths. Usually, the linewidth is determined from the power density spectrum, i.e. the squared amplitude spectrum. The linewidth in the amplitude spectrum and the corresponding power density spectrum differs by a factor \(\sqrt{2}\) if a Gaussian-shaped spectrum is given. The shape of the spectrum is unknown. Moreover, the linewidths are in the range of the frequency resolution and are only estimates. Therefore, it is of secondary importance from which spectrum the linewidth is determined. Due to the unknown spectral shape \({L}_{{{{{{\rm{c}}}}}}}=c/\triangle \nu\) with coherence length \({L}_{{{{{{\rm{c}}}}}}}\), speed of light \(c\) and linewidth \(\triangle \nu\) is used to determine an estimation of the coherence length.

Despite choosing the name single-mode phase evolution for the phase evolution at one point in time over consecutive sweeps we could not measure single modes. The consecutive sweeps of an FDML laser with a sweep span of 40 nm comprise 17.5 million modes. The 0.7 nm and 21 ns long beat signals include 307,000 laser modes. Therefore, the 6.25 ps gate width of the fast oscilloscope contains more than 100 comb lines.

All-mode phase evolution

The data in Fig. 4d, e were generated as follows. The beat signals were overlaid for the evaluation of the single-mode phase evolution, but the amplitude not at one time point but for all data points of a beat signal was compared, generating as many single-mode phase evolution plots as data points were present. The edges of the beat signal (around 5% on each side), where the amplitude starts to decrease, are not evaluated. Each was Fourier transformed. The frequencies with the maximum amplitude in every spectrum were detected by fitting a parabola through the highest data point and its neighbours on both sides. The frequency resolution of around 5 kHz generates numerical artifacts, which are explained in the Supplementary Note 4. The frequencies with the maximum amplitude (besides the zero frequency part) are then plotted over time. Figure 4e shows all-mode phase evolution plots for different datasets and laser parameters. Here the percentage of a full beat signal is used as x-axis since the beat signals are of different lengths. In measurements 1 and 2 the FDML laser had a bandwidth of 40 nm and the CW laser had a wavelength of 1305 nm. Measurement 1 originates from the dataset presented in Figs. 2 and 3, where approximately 3000 data points within the range from 157.5 ns to 176.5 ns were analysed. In measurements 3 and 4 the FDML laser had a bandwidth of 80 nm around 1290 nm and the CW laser had a wavelength of 1290 nm. Therefore, the beat signal is shorter and contains approximately 1300 data points. The sweep range in measurements 3 and 4 is 80 nm, but the total sweeps also last approximately 1 µs. The beat signal is evaluated over 8 ns. The visible sweep in measurements 1 and 2 is approximately 21 ns long and evaluated over 19 ns due to the shorter sweep range of 40 nm. The graphs in Fig. 4d, e present the phase change with each roundtrip over the beat signal (relative phase accumulation), whereas 205.5 kHz corresponds to π. However, a specific value cannot be determined as the exact frequency of the CW laser is unknown. Note, all-mode phase evolution includes all modes within the beat signal, not all modes within a sweep. The number of the modes is limited by the beat frequency bandwidth.

Simulations

Simulations are based on the model from Schmidt et al.²⁶ with a centre sweep wavelength of 1290 nm and a sweep bandwidth of 40 nm similar to the data in Fig. 2. 1000 consecutive roundtrips were analysed extracted from a laser setup operated in the ultra-stable or sweet-spot regime as in the case of the experimental data and the results compared to lasers operating beyond the ultra-stable regime. Here the intensity trace is distorted by high-frequency fluctuations^26,29,32. The amount of dispersion in the fibre delay cavity was varied in order to take into account an imperfect dispersion compensation. The fibre dispersion is a dominant noise source in FDML lasers²⁹ and serves here as a model for non-synchronized operating conditions. As shown in the publication from Schmidt et al.²⁶ the laser can still run in the ultra-stable regime up to a threshold of group delay in the fibre cavity due to the dynamics induced by the swept Fabry Pérot filter. Since timing delays caused by the residual dispersion in the fibre cavity accumulate over time, this phenomenon strongly indicates that the optical phase is locked in the ultra-stable regime as shown by the measurement results of Figs. 2–4.

The beat signal \({E}_{{{{{{\rm{beat}}}}}}}(t)\) of Eqs. (1) and (2) which is proportional to the optical power incident on the photodetector, i.e. \(\propto {|{E}_{{{{{{\rm{CW}}}}}}}\left(t\right)+{E}_{{{{{{\rm{FDML}}}}}}}\left(t\right)|}^{2}\), can be computed to be

We choose \(t\in [0,{T}_{{{{{{\rm{R}}}}}}}]\) with the roundtrip time \({T}_{{{{{{\rm{R}}}}}}}\) and express arbitrary points in time by adding the roundtrip number \(n\) times the roundtrip time. From Eq. (3) it follows that if the FDML laser has a periodically varying instantaneous frequency with period \({T}_{{{{{{\rm{R}}}}}}}\), apart from a constant phase accumulation per roundtrip\(\,{\varphi }_{{{{{{\rm{FDML}}}}}}}(t+n{T}_{{{{{{\rm{R}}}}}}})=n\bullet {\varphi }_{{{{{{\rm{CEP}}}}}}}\) with the carrier-envelope phase slip\(\,{\varphi }_{{{{{{\rm{CEP}}}}}}}\), the beat signal is \(\propto {{\cos }}\left(n{\varphi }_{{{{{{\rm{beat}}}}}}}+{\varphi }_{{{{{{\rm{const}}}}}}}\right).\)

Here

and

when tracking the beat signal at an arbitrary point in the sweep \({t}_{0}\) over consecutive roundtrips as depicted in Fig. 3.

This simple model based on idealized signals is able to explain the measured beat signal from Fig. 4a. Note that the measured beat signal also contains a contribution of the CW laser, which prohibits a direct observation of the CEPS of the FDML laser from our measurement. Our simulation confirms the constant phase accumulation of the optical field over the entire sweep, which is an extension to Fig. 4d where a stable beat signal was presented in a 19 ns window of the 1 µs long sweep. The phase accumulation \({\hat{\varphi }}_{{{{{{\rm{CEP}}}}}}}\) of the FDML laser extracted from our simulation is shown in Fig. 5a for different amounts of residual dispersion in the fibre cavity. The residual dispersion is characterized by the maximum group delay difference between the fastest and slowest wavelength in the fibre cavity. Here we traced the phase of the optical field in the swept-filter reference frame¹³ over 100 roundtrips and computed the phase difference between consecutive roundtrips at each point in the entire sweep, where a maximum deviation from a constant rate was identified to be below 0.05 rad for all traces in the ultra-stable regime. The optical phase in the swept-filter reference frame contains all phase fluctuations induced by the laser components apart from constant contributions of the group-phase offset³⁷, the Fabry Pérot filter mirrors³⁸ or the CW laser as mentioned above. This means that the absolute phase accumulation rate φ_beat as measured in Fig. 4 cannot be extracted from the simulation but is equal apart from a constant offset to \({\hat{\varphi }}_{{{{{{\rm{CEP}}}}}}}\) in Fig. 5a. The fundamental advantage of the swept-filter reference frame is the possibility to analyse the residual noise in the entire sweep bandwidth. This is impractical for the full field due to the large time-bandwidth product in the order of ≈ 10⁷which is reduced by three orders of magnitude in the reference frame. For comparison to the ultra-stable case, the average phase accumulation rate for a residual dispersion of 1000 fs and 1900 fs is included in Fig. 5a, when the intensity trace of the laser is distorted by non-stationary high-frequency intensity fluctuations, i.e. when not all of the modes are locked. The evolution follows the parabolic phase accumulation induced by the fibre dispersion, which can be extracted from the model of Schmidt et al²⁶., and is not compensated in this regime. Single sweet-spots still exist in the intensity trace due to single wavelengths synchronized with the sweep rate as shown by Pfeiffer et al.²⁹ and are visible as constant lines in Fig. 5a.

Since an ideal periodic signal possesses a comb spectrum in the Fourier domain, we Fourier transformed 1000 consecutive roundtrips, equivalent to an observation time of 2.4 ms, of an ultra-stable as well as a non-dispersion-compensated laser. We analysed the spectrum of the full field in a small window around 1305 nm, corresponding to the wavelength of the CW laser in Figs. 2–4, due to the limited simulation bandwidth as discussed above. Therefore, we have further verified our results at various positions in the sweep. The spectra around 1305 nm are presented in Fig. 5b, c. Even in the non-dispersion-compensated case in Fig. 5c, which has been shown to be a stationary lasing regime¹³, a quasi-mode structure is observed. The width of these modes can significantly be reduced in the ultra-stable regime in Fig. 5b to near-ideal comb lines (see inset in Fig. 5b), consistent with the stable phase slip in the experiment demonstrated in Fig. 3.