1 Kent Academic Repository Full text document (pdf) Citation for published version Toadere, Florin and Stancu, Radu.-F. and Poon, Wallace and Schultz, David and Podoleanu, Adrian G.H. (2017) 1 MHz Akinetic Dispersive Ring Cavity Swept Source at 850 nm. IEEE Photonics Technology Letters, 29 (11). pp ISSN DOI Link to record in KAR Document Version Author's Accepted Manuscript Copyright & reuse Content in the Kent Academic Repository is made available for research purposes. Unless otherwise stated all content is protected by copyright and in the absence of an open licence (eg Creative Commons), permissions for further reuse of content should be sought from the publisher, author or other copyright holder. Versions of research The version in the Kent Academic Repository may differ from the final published version. Users are advised to check for the status of the paper. Users should always cite the published version of record. Enquiries For any further enquiries regarding the licence status of this document, please contact: If you believe this document infringes copyright then please contact the KAR admin team with the take-down information provided at
2 PTL MHz akinetic dispersive ring cavity swept source at 850 nm Florin Toadere 1, Radu-F. Stancu 1, Wallace Poon 2, David Schultz 2, Adrian Podoleanu 1 Abstract - A fast dual mode-locked akinetic optical swept source at 850 nm central wavelength is presented using a dispersive cavity. We demonstrate that single mode fiber can be successfully used as dispersive element to induce mode locking. A first locking condition is imposed by driving the optical gain at a high frequency, to induce mode locking, while a second locking condition involves sweeping at a rate close to resonance value. In this regime, using the same fiber length in the loop, sweeping rates of 0.5 MHz and 1 MHz are demonstrated with proportional reduction in the tuning bandwidth. The axial range of the swept source was evaluated by scanning through the channeled spectrum of a Michelson interferometer. Index Terms Semiconductor optical amplifier, laser mode locking, ring laser, optical fiber dispersion, radio frequency. T I. INTRODUCTION he majority of commercial OCT systems for retina imaging operate in the nm band using spectrometer based OCT principle , . There is a trend in equipping commercial systems for ophthalmology with swept sources, due to longer axial range and better sensitivity, however fast sweeping rates are available at longer wavelengths only. At shorter wavelength, transversal resolution is better and the 800 nm band is also suitable for oxygenation measurements. Therefore, a continuous interest exists in developing swept sources at a 850 nm central wavelength. Development of fast swept sources (SS) focused on optomechanical principles of tuning applied to a closed loop laser cavity. To exemplify, the following devices were employed: a Fabry-Perot filter , a polygon filter , or a micro electrical mechanical scanning (MEMS) device [6,7]. These optomechanical elements can exhibit hysteresis and are strongly influenced by electrical instabilities. An alternative in the technology of swept sources for OCT consists in the elimination of the mechanical tuning elements from the laser cavity , to achieve akinetic operation. Another akinetic principle is based on dispersion tuning [9-12]. We have demonstrated recently an improvement to this concept, represented by dual resonance excitation . A first resonance condition is imposed by driving the optical gain at a multiple of the cavity resonance frequency, f R, to induce mode-locking. The second mechanism Paper submitted on: 01/11/2016. Radu-F. Stancu, Florin Toadere, Adrian Gh. Podoleanu are with the Applied Optics Group, School of Physical Sciences, University of Kent, Canterbury, United Kingdom, CT2 7NH ( Wallace Poon and David Schultz are with the East Kent Hospital University NHS Foundation Trust. A. P. was is responsible for generating sweeping at a rate close to f R, more exactly at values detuned from f R . So far, this dual resonance mechanism was proven at wavelengths longer than 1 m. In this paper, the concept of dual mode-locking and dispersion tuning is extended to the 850 nm band. In comparison with previous implementations, we demonstrate that the dispersion of single mode fiber at 850 nm, used as the necessary delay element, is sufficient to act as dispersive element for the principle illustrated. At longer wavelengths, other dispersive elements have been reported, such as dispersive compensating fiber [9,10,13] or chirped fiber Bragg gratings . MHz sweeping is reported with sufficient narrow linewidth (90 pm) and tuning bandwidth over 10 nm. II. DUAL MODE-LOCKING The operation of mode-locking dispersion ring cavity akinetic swept source (AKSS), sweeping at hundreds of khz was presented in several reports [9-13]. The resonance frequency of the ring laser cavity is defined as: c fr (1) nl where: c represents the speed of light in vacuum, n is the refractive index of the cavity and L is the laser cavity length. f R denotes the spacing between adjacent modes in a resonant cavity. Mode locking is induced by modulating the semiconductor optical amplifier (SOA) at a frequency f m0, that is a multiple, N, of the frequency f r, e.g. Nf R. Mode-locking allows obtaining ultrashort laser pulses at the SOA output. Thus, SOA operates at a wavelength that fulfills the modelooking condition. The wavelength tuning range is given by: n 0 fm (2) cdfm 0 where f m0 is the central mode-locking frequency, n 0 is the refractive index at central optical frequency, D is the dispersion parameter of the ring, and f m represents the change in modulation frequency. When f m reaches the resonance frequency, the maximum tuning range max is achieved: 1 max (3) D Lf m 0 The frequency of the RF signal is changed within the bandwidth f m using a voltage controlled oscillator (VCO). When the frequency of the signal driving the VCO is swept at f s = f R, the VCO s spectrum is described by a comb of frequencies f m0 ± f R, supported by the Royal Society Wolfson Research Merit Award and by the BRC-UCL at Inst. of Ophthalmology London; F.T was supported by Univ. of Kent; R.-F. S. was supported by the Brilliant Club Researchers in Schools Programme; F.T. and R-F. S. have equally contributed to the paper.
3 PTL f m0 ± 2f R, f m0 ± 3f R, and so on. Under these conditions, each component of the comb differs from f m0 by a multiple of f R and therefore all comb components determine the same laser optical frequency, repeated at f R. Therefore, to obtain optical tuning, the frequency of the signal driving the laser has to be detuned from resonance. For a detuning f R, with f s = f R ± f R, the spectrum of the VCO is a comb spectrum consisting in f m0, f m0 ± f R ± f R, f m0 ± 2f R ± 2 f R, f m0 ± 3f R ± 3 f R and so on. The step f r of the radiofrequency determines a change in wavelength. In such case, the output bandwidth is given by: (4) where C m is a coefficient and F represents the frequency tuning range of the VCO. The step change in wavelength can be assimilated to the linewidth of the optical emission. The optical configuration of the AKSS experimented is shown in Fig. 1. It uses a SOA model SOA-372, operating at 850 nm, maximum current 200 ma and 3 db bandwidth up to 40 nm. The SOA is placed in a laser system mount model HS501, and it is driven by a Thorlabs LDC 210C power supply. Temperature stability is ensured through thermo-electric cooling provided by a Thorlabs TEC 200C driver (sensor TH 20kΩ). The SOA is delimited by two polarization insensitive isolators IO-F-SLD which ensure unidirectional lasing. Output power is drawn out via a 50/50 FOC Photonics coupler with the central wavelength 850 nm, with 50% of power reinjected in the cavity. Fig. 1. Akinetic swept source schematic diagram The output spectra of the SS configuration are measured using a Yokogawa AQ6375 optical spectrum analyzer (OSA). The coherence length of the source is evaluated by feeding a balanced interferometer, equipped with a spectral delay line for dispersion adjustment, terminated on a 50/50 SIFAM coupler with the central wavelength 850 nm, ending on a New Focus balanced detector model 1807, 80 MHz, nm bandwidth, whose output is displayed with a LeCroy Wave Runner 104 MXi-A 1GHz oscilloscope, at a sampling rate of 10 GS/s (Gyga Sample points/ second). In dynamic regime, a ZX CA-S+ Mini Circuits low noise VCO and a ZHL-42W Mini Circuits radio frequency amplifier are used to drive the SOA. The VCO is driven by a ramp signal generated by an Agilent 81160A radio frequency signal generator (RFSG), with adjustable amplitude, frequency and offset. The offset of the ramp signal determines the central mode-locking frequency f mo generated by the VCO and its amplitude sets the tuning bandwidth F of the VCO. The repetition frequency of the ramp signal determines the tuning rate f s. Then the VCO s signal is amplified by the radio frequency amplifier before it is delivered to SOA. By changing the frequency of the VCO signal, the SOA optical frequency is swept. Empirically it was noted that ramps driving the optical frequency from high to low leads to a larger tuning bandwidth and a better signal to noise ratio than ramps driving the optical frequency from low to high. This is in line with previous observations in FDML lasers . III. EXPERIMENTAL RESULTS A Hewlett Packard 8648C RFSG, operating in the 9 khz MHz frequency band, was employed in the measurements performed in the static regime (manually adjusting the modelocking frequency). The amplitude of signal at the SOA driver input is 15 V at 250 MHz and 12 V at 1 GHz. At larger frequencies, although the generator output was increased, no change was noticed in the spectrum, suggesting that the power amplifier worked in saturation regime. However, we cannot measure the exact current swing through the SOA due to limitation of our electronic bandwidth (photodetector and oscilloscope). Inside the driver, there is a serial resistance of 33 in series with the SOA, determining a load of approximately 50. The cavity dispersion was adjusted by modifying the length of the resonant cavity. Thus, four types of single mode fibers are tested as listed in Table 1. For all four configurations the resonance frequencies are listed in the 3 rd column of Table 1. As expected, the larger the length of the cavity, less is the resonance frequency. The fourth and fifth columns list the optical bandwidth and respectively, the tuning of the f m0 range to generate the output bandwidth, for a mode-locking frequency f mo = 1 GHz. The widest tuning bandwidth was achieved when using fiber F2. Table 1. Static regime of the resonant cavity Fiber type Cavity length [m] Resonance fr [khz] max [nm] f s [khz] F1: SM 780/125 A F2: Thorlabs FS SN-4224 F3: Lightwave Tech Inc., F1506C F4: Ensign Bickford SMC A0820B Fig. 2 (left) illustrates the static bandwidth for two different values of the RF frequency, f m0. By varying the RF to either extremity of the RF bandwidth, two optical frequency components are seen. This proves that f m reaches the resonance frequency condition, that lead to Eq (3). Fig. 2. Left: Optical spectrum for f m0 = 250MHz ± 115 khz (solid line) and f m0 = 2GHz ± 125 khz for (dashed line); Right: Maximum tuning bandwidth (blue solid line) and the radio frequency tuning range F (dashed red line) necessary to achieve = 4 nm versus the frequency of the mode-locking frequency, f m0.
4 PTL Table 2 presents several measurements of the wavelengths and linewidths obtained in static regime employing a 400 m ring cavity. The central mode-locking frequency f m0 inducing the mode locking is varied from 250 MHz to 2 GHz. For each value of f m0, its frequency was statically modified and the optical frequency determined by the change was followed in its variation using the optical spectrum analyzer. The maximum bandwidth values obtained, max, are listed in the second column, while the linewidths measured are shown in the third column. The number of points in the spectrum are calculated in the fourth column, by dividing the maximum optical tuning bandwidth to the value of linewidth. Because this is measured for a static condition, where the frequency of the VCO is held fixed, we refer to such linewidth as static, st This is the minimum achievable linewidth and should be distinguished from the step used in Eq. (4) in dynamic regime. In the fifth column, the values of the dispersion parameter D are calculated using Eq. (3). These give an average value D = 199 [ps/nm/km]. The RF deviations that created these optical tuning bandwidths are mentioned in the sixth column. Table 2. Measured parameters of the AKSS employing a 400 m Lightwave Technologies, F1506C fiber ring cavity f m0 [MHz] max [nm] st [nm) max/ st Then we maintained the maximum frequency of 2 GHz of the signal applied to the SOA and changed the fiber. The narrowest static linewidths st of 70 pm are achieved using F3 and F4. Larger static linewidths of 130 pm and 120 pm are obtained using the fiber F1 and, respectively, F2. For the linewidth measurements, the OSA was adjusted on a resolution of 20 pm. These measurements refer to the width of envelope emission over many optical modes, impacting both the sensitivity decay with the path difference in the interferometer and what can be effectively measured with an OSA. Fig. 3. Amplitude versus radio frequency applied to VCO D [ps/nm/km] f m [khz] Understanding the RF spectrum is important, as each RF component in the spectrum determines an optical frequency, as described by equation (4). At 1500 khz tuning rate, only a single peak is left in the RF spectrum in Fig. 3. According to the theory of frequency modulation, the index of modulation is the ratio of F/f m. For the data in Fig. 2 (left), using 60 MHz/f m, suggests that even at 1 MHz, the index is larger than 1. This means that the radio frequency spectrum consists of more components than J1 (integer number Bessel function first order). At 1.2 MHz these multiple components can be resolved by the RFSA. According to Table 2, when using the VCO close to 1 GHz, if the RF is varied by 230 khz, then a tuning bandwidth of 13 nm should be achievable. Next, the dynamic regime operation of the source is characterized. A sinusoidal wave produced by the RSFG is applied to the VCO and the RF spectrum is measured using an Agilent RF Spectrum analyzer. The VCO operation is tested with signals of repetition frequency between 100 khz to 1500 khz and amplitude kept constant at 5 V. According to the VCO data sheet, this amplitude should produce a tuning of 100 MHz. Dynamically, the laser optical output is tuned according to two interleaved mechanisms, as previously communicated [8, 9]. A first mechanism is that of dispersive tuning. The second tuning mechanism involves resonance tuning using the cavity roundtrip; the tuning must be slightly detuned from the cavity resonance. We show the effects obtained for seven such steps, where each step is f R = 500 Hz. For f S = f R 500 Hz, the VCO frequency contains the following components: f m0 to f m0 f R 500 Hz, f m0 2f R 1 khz, f m0 3f R 1.5 khz and so on, in the MHz range, in steps of khz. The emitted spectrum consists of a comb having a linewidth of 80 pm and a repetition rate of khz. For a 3.8 Vpp signal driving the VCO and a laser driving current of 50 ma the output power of the AKSS is 1.6 mw. The 3.8Vpp determines a VCO tuning bandwidth of 60 MHz. Fig. 4. Optical spectra at the SS output measured for seven values of detuning of the sweeping frequency starting from resonance By modulating the VCO at sweeping rates of 999 khz, khz, 998 khz, and so on, down to 996 khz, in steps of 0.5 khz, the measured output bandwidths are: 1.3 nm, 2.5 nm, 4.5 nm, 7.5 nm, 10 nm, 11 nm and, respectively 13.5 nm. This is illustrated in Fig. 4. For an OPD = 50 µm, for each step of detuning the frequency of the VCO by 500 Hz, a new peak is observed in each channeled spectrum at the interferometer output (seen on the oscilloscope screen, not shown here) in addition to the previously existing peaks. While the tuning bandwidth increases, the optical power decreases, due to driving the laser away from the round trip resonance. The effects of tuning on the output optical power are better documented in Fig. 5. Fig. 5 (left) illustrates the output power versus amplitude of the ramp signal applied to the VCO s input for a detuning of f R = 3 khz when operating the AKSS dynamically at a 996 khz sweep rate. The larger the
5 PTL amplitude of the ramp driving the VCO, the larger the tuning bandwidth. On the other hand, a decrease in power is noticeable. Fig. 5 (right) depicts the optical tuning bandwidth and the output power versus the repetition rates around the central resonance of 999 khz. The larger the detuning from the resonance frequency, the wider the bandwidth and the lower the optical power. A maximum power of 1.6 mw is achieved at the output, without using a booster. Furthermore, the AKSS output was connected to a Michelson interferometer and used the following formulae to determine the useful power of the swept source ( P SS ) and the ASE power ( P ASE ), based on the optical power measured in the reference and, respectively, object arm: Pobj O PSS PASE, Pref R PSS PASE, where: O represents the object arm coefficient, R represents the reference arm coefficient. From the two equations above, the interference power is obtained as: int ORPSS. Using the method described above, it was concluded that the useful swept source output power represents 20% of the total output power. Fig. 5. (left) Output power (dashed blue line) and tuning bandwidth (red solid line) versus the amplitude of the ramp signal applied to the VCO input for f R = 3 khz; (right) tuning bandwidth (red solid line) and optical power (dashed blue line) versus the AKSS repetition rate Fig. 6. Amplitude decay of the photo-detected signal with OPD (roll off) for seven detuning values of the sweeping frequency f S detuned from f R = khz. The sensitivity S of the interferometer, in case is used for reflectivity measurement, is calculated as, where A OPD is amplitude of the FFT peak for an OPD close to 0, while A noise represents the amplitude of the noise floor measured outside the peak . S = 88.5 db is obtained for an OPD value of 0.1 mm. The shape for the spectrum emission is assumed Gaussian and using the OPD 6dB worked out from Fig. 6 of 1 mm (obtained for 2 khz detuning, i.e. the 998 khz curve), a dynamic linewidth of = pm results. This is slightly larger than the static linewidth measured in Table 2, of 80 pm. As the sweeping speed increases, the A-scans showed a decrease in the axial range (OPD). This can be explained by a decrease in the swept source coherence length with the sweeping speed, due to the decrease of roundtrip in the cavity, as it was previously shown in . However sufficient amplitude is obtained up to 6 mm for less detuning, as shown by the curves for and 999 khz in Fig. 4 and Fig. 6. IV. CONCLUSION In this paper, an 850 nm central wavelength AKSS swept at 1 MHz, with over 1 mw power is demonstrated. At a sweeping rate of khz, determining a 4 nm tuning bandwidth, the OPD for 6 db decay in the interferometer signal is 1 mm. 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