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We demonstrate simultaneous repetition rate multiplication of laser pulses at multiple wavelengths using the spectral elimination approach. The phase coherence between the pulses is preserved while the aggregate bandwidth can be significantly enhanced. The repetition rates of pulses from both a mode-locked fiber ring laser and a pair of gain-switched DFB laser diodes have been multiplied to over 18 GHz per wavelength using the same setup. The key element is an all-fiber, polarization independent birefringence loop mirror comb filter. The broad transmission peaks of the filter also allow a large tolerance in the drift of the input wavelengths and repetition rates. A detuning range of 1.2 GHz is observed, corresponding to 13% of the input frequency.
©2005 Optical Society of America
1. Introduction
High-repetition rate optical pulse sources are important for applications in broadband communications and microwave photonics [1–2]. However, the direct generation of such sources is usually limited by the requirement of high-speed electronic circuitry and components. Hence, external repetition rate multiplication of laser sources is of great interest in reducing the cost and increasing the flexibility of their operation. To enhance the repetition rate, different types of fiber multiplexers that combine two or more time-interleaved optical branches have been proposed [2–3]; however, the schemes require precise trimming of the fibers to obtain accurate delay times among the branches. Phase coherence will also be lost unless an interferometric stabilization scheme is used. Recently, several approaches have been proposed for pulse repetition rate multiplication. Examples include the application of fractional Talbot effect in a linearly tunable chirped fiber Bragg grating [4], the use of a number of uniform fiber Bragg gratings [5], the use of coupled Fabry-Perot resonators [6], and spectral elimination by Fabry-Perot filter/etalon or wavelength-division multiplexers (WDM) [7–9]. Although high finesse filters and WDM pairs are relatively lossy elements, the spectral elimination approach can maintain the phase coherence between the multiplied laser pulses.
In this paper, we demonstrate a passive, all-fiber approach to increase the pulse repetition rate based on spectral elimination using a birefringence loop mirror filter (LMF) [10–11]. The spectral characteristics of the laser pulses are modified to allow the conversion into a higher repetition rate pulse train. With the use of a LMF, the birefringence is only introduced in a single segment of polarization maintaining fiber (PMF). The LMF is superior to the Mach-Zehnder interferometer in a way that we only have to determine one fiber length but not two. For a LMF with a free-spectral range (FSR) of 10 GHz, the 3 dB transmission bandwidth is 0.04 nm and the peak-to-notch contrast ratio is over 25 dB. In comparison, for a fiber based Fabry-Perot filter with a FSR of 10 GHz and a finesse of 10, the 3 dB transmission bandwidth is 0.008 nm and the peak-to-notch contrast ratio is only around 16 dB. Besides, the LMF can offer an operation bandwidth of over 100 nm, while the operation range of a Fabry-Perot filter may be limited by the coatings of the mirrors. Owing to the periodic transmission characteristic of the LMF over a wide spectral range, simultaneous repetition rate multiplication for different WDM channels becomes possible when the channel spacing is a multiple of the filter comb spacing. This feature is particularly attractive for use with multi-wavelength laser pulses generated from both semiconductor and fiber lasers [12–13]. By taking advantage from the broad transmission peaks of the LMF, a large tolerance towards both the wavelength shift and the input repetition rate variation can be obtained. In addition, since the transmission characteristic of the LMF is polarization independent, no polarization control is needed at the input and the polarization property of the laser source is preserved. Pulse repetition rate multiplication is demonstrated for both a mode-locked fiber ring laser and a pair of gain-switched semiconductor laser diodes to produce pulses over 18 GHz.
Our approach makes use of the relation between the pulse repetition period (τp) and the spacing of the optical spectral lines (Δυm), as described by
By eliminating every other components of the spectral comb without destroying the coherent phase relationship of the remaining components, the repetition rate can be doubled, as schematically illustrated in Fig. 1.
2. Experimental setup
Figure 2 shows a birefringence loop mirror filter (LMF) that is used as the key element for repetition rate multiplication. The LMF consists of a 50:50 coupler, a polarization controller (PC), and a polarization maintaining fiber (PMF). The PC controls the peak-to-notch contrast ratio of the filter, while the PMF defines the comb spacing, which is given by
in terms of wavelength and frequency, respectively. Here, Δn, L, and v are the birefringence, the length, and the speed of light in the PMF. The comb spacing remains essentially constant over the range of operation.
Fig. 2. Repetition rate multiplication of multi-wavelength pulses. PMF: polarization maintaining fiber; PC: polarization controller. Yellow stripes correspond to the transmission wavelengths of the LMF.
The operation principle of the LMF is described as follows. The input beam is split into two counter-propagating branches by the 50:50 coupler. The PC is set to produce a pure rotation of π/2 for light coming from both directions. After traveling in the PMF loop, the two light branches develop a phase difference and interfere at the coupler. The phase difference, and hence the light transmission, is periodic with respect to the inverse of wavelength. The transmission function is given by
The width of the transmission peak is relatively broad and thus the LMF is insensitive to slight drifts of the input wavelength. Also, a relatively large tolerance to the variation of the input repetition rate can be obtained.
In Fig. 2, we consider a low repetition-rate multi-wavelength input source containing pulses at three different center wavelengths and aligned with the transmission peaks of the LMF. The LMF is set to have a comb spacing equals to twice of the spacing between the spectral lines. Thus, every other spectral lines are being blocked by the transmission notches of the LMF. As a result, the output will contain lines with a spacing doubled that of the input pulsed source and the repetition rate is doubled. Hence, simultaneous repetition rate multiplication can be achieved for a source containing inputs at different wavelengths.
3. Results and discussion
To demonstrate the principle of repetition rate multiplication, a 9.3 GHz pulsed train from an actively mode-locked fiber ring laser (ML-FRL) is launched into the LMF. The birefringence of the PMF is 2.98×10-4 and the length is 54.2 m, resulting in a comb spacing of 0.15 nm (18.7 GHz at ~1550 nm). The temporal profile and the optical spectrum of the pulsed source are shown in Fig. 3(a) and 3(b), respectively. The wavelength spacing of the components is 0.07 nm and thus a LMF with 0.15 nm comb spacing is used to filter out every other components. The comb position is slightly tuned to control the suppression of the undesired spectral components. After passing through the LMF, a 18.6 GHz pulsed source is obtained at the output, as shown in Fig. 3(c). Figure 3(d) shows the corresponding optical spectrum. The wavelength spacing of the 18.6 GHz pulsed source becomes 0.14 nm. The result shows that the repetition rate of the ML-FRL has been successfully multiplied from 9.3 to 18.6 GHz based on spectral elimination with the LMF.
Fig. 3. 9.3 GHz to 18.6 GHz repetition rate multiplication of ML-FRL pulses. (a) 9.3 GHz input pulses; (b) Optical spectrum of the input; (c) 18.6 GHz multiplied pulses; (d) Optical spectrum of the multiplied output.
Owing to the non-optimized adjustment of the LMF, the short wavelength components experience a higher suppression than the long wavelength components, as shown in Fig. 3(d). In order to obtain a more uniform suppression over the spectrum, the LMF should be adjusted such that the center part of the spectrum will have the largest suppression. The difference in the spectral envelopes between the input and the output is caused by a mismatch in the LMF comb spacing and the frequency of the input source. The effect is stronger for a narrow input pulse that is associated with a wide spectral width. The difference in the envelopes will result in a change of the pulse shapes but will not affect the multiplied repetition rate.
To demonstrate simultaneous repetition rate multiplication of pulses at different center wavelengths, two DFB laser diodes are gain-switched at 9.0 GHz and are used as multi-wavelength input pulsed sources. The two DFBs are thermally tuned to emit light at center wavelengths of λ1=1548.08 nm and λ2=1549.10 nm. The wavelengths are chosen such that their separation is equal to a multiple of the LMF comb spacing. Figures 4(a) and 4(b) show the combined 9.0 GHz pulsed source and its optical spectrum, respectively. The difference in the spectral profiles of the two sources is attributed to the different modulation characteristics of the DFBs. The wavelength spacing of the spectral components is about 0.07 nm, corresponding to a 9.0 GHz pulsed source. The input time and amplitude jitter is obtained by integrating the phase noise power spectral density from 100 Hz to 10 MHz in the RF spectrum. A value of 117 fs is obtained. After passing through the LMF, output pulses at 18.0 GHz are obtained as shown in Fig. 4(c). The corresponding spectrum is depicted in Fig. 4(d). Every other spectral components of both pulsed sources have been filtered out with a suppression >20 dB, leading to simultaneous pulse repetition rate multiplication. The measured time and amplitude jitter of the multiplied output is 120 fs. The close proximity of the jitter values between the input and the output indicates that the multiplication process using the LMF does not introduce much additional noise.
Fig. 4. 9.0 GHz to 18.0 GHz simultaneous repetition rate multiplication of pulses at multiple wavelengths. (a) 9.0 GHz dual-wavelength input pulses; (b) Optical spectrum of the dual-wavelength input; (c) 18.0 GHz multiplied pulses; (d) Optical spectrum of the multiplied output.
The RF spectrum of the 9.0 GHz dual-wavelength input pulse source has been measured and the result is shown in Fig. 5(a). A strong frequency component at 9.0 GHz is observed together with components at 18.0 GHz, 27.0 GHz, and 36.0 GHz, corresponding to the 2 nd,3 rd, and 4 th harmonics of the fundamental frequency. After the 9.0 GHz pulsed source is multiplied by the LMF, a 18.0 GHz pulsed train is obtained and the RF spectrum is shown in Fig. 5(b). The frequency component at 9.0 GHz has been dramatically suppressed by over 40 dB and that at 27.0 GHz has also been suppressed by over 20 dB. The strongest frequency component now appears at 18.0 GHz, indicating a multiplied 18.0 GHz pulsed source. The incomplete suppression of the 9.0 GHz and 27.0 GHz components will result in a weak amplitude envelope of 9.0 GHz in the multiplied source. From our result, the multiplied output shows a very weak 9.0 GHz component with a magnitude of 30 dB below the 18.0 GHz component. Hence, the 9.0 GHz envelope is almost undetectable at the pulsed output. The measured pulse-to-pulse variation of the amplitude is 1.67 %. Although the suppression of the undesired components is incomplete, the 40 dB suppression of the 9 GHz fundamental component is sufficient for generating a high-repetition-rate multiplied source with a relatively uniform amplitude.
To gain a better understanding on the multi-wavelength operation of the setup, the two wavelengths at both the input and the output ports are individually filtered out using a 0.5 nm optical filter. The input pulse trains of the individual wavelengths are shown in Fig. 6(a) and (b), while the multiplied pulse trains are shown in Fig. 6(c) and (d). Since the two wavelengths are very close to each other, the filtered output still contains certain residue portion of the rejected wavelength but with a suppression higher than 15 dB.
Fig. 6. Temporal profiles of components at individual wavelengths in the dual-wavelength repetition rate multiplication experiment. (a) 9 GHz input pulses at 1548.08 nm; (b) 9 GHz input pulses at 1549.10 nm (c) 18.0 GHz multiplied pulses at 1548.08 nm; (d) 18.0 GHz input pulses at 1549.10 nm
It is worth mentioning that although the two pulsed sources are synchronized in our experiment, the setup can work equally well for non-synchronized pulse trains at multiple wavelengths. Synchronization is not needed in our scheme because the temporal property is determined by the modification of the pulse characteristics in the spectral domain.
As illustrated from the above experiments with fiber and semiconductor lasers, the same length of PMF can be used in the multiplication of pulses at both 9.3 GHz and 9.0 GHz. The wide range of allowable input frequency is a result of the relatively broad transmission peaks of the LMF. Hence, a large tolerance is obtained in the mismatch between the input frequency and the LMF comb spacing. However, the quality of the multiplied output will be slightly degraded when the input pulse repetition rate is detuned from the optimized frequency. In the following, we study the tolerance systematically by tuning the input frequency and observe the change in the suppression of the undesired spectral components.
The suppression of the components is plotted against the input pulse repetition rate from 8.5 to 10.0 GHz, as shown in Fig.7. Using a LMF with 0.15 nm comb spacing, the optimized input frequency to achieve a maximum suppression is about 9.2 GHz. We observe that the multiplication scheme works well for a repetition rate ranging from 8.8 to 10.0 GHz. Within this range, the deviation in the pulse separation of the multiplied output is less than 5%. The 1.2 GHz detuning range corresponds to 13% of the optimized input frequency.
In principle, the quality of multiplication can also be verified by measuring the input and the output pulse shapes. Ideally, the pulse shape should remain unchanged. As observed from Fig. 3(d), the spectral envelope is changed and it will result in a distortion of the output pulse as shown in Fig. 3(c). In contrast, no apparent change in the spectral envelope is observed in Fig. 4(d). Consequently, the pulse shape is expected to be preserved. However, owing to the limited resolution of the oscilloscope and overlapping of the multiplied pulses, the pulse shape cannot be precisely determined. To better judge the quality of the multiplied pulses, an all-optical sampling technique with a sub-picosecond time resolution can be used to measure the temporal profile of the output.
Although the scheme for multiple input wavelengths is demonstrated with a multiplication factor of 2 to yield an output at 18 GHz, a higher multiplication factor is possible by cascading LMFs with different comb spacings, or by adopting higher order birefringence comb filters [11]. Depending on the number of remaining spectral components, different multiplication factors can be obtained. Programmable operation can also be realized by cascading LMFs of different comb spacings with optical switches [14], allowing multiplication to be performed for different input pulse repetition rates. Applying electrical [15] or optical control [16] of birefringence in the LMF, fast tuning of the repetition rate can also be made possible by modifying the spectral characteristics of the pulses.
4. Conclusion
Multiplication of repetition rate has been experimentally demonstrated by spectral elimination with a birefringence loop mirror filter. By eliminating every other components of the spectral comb without destroying the coherent phase relationship of the remaining components, the pulse repetition rate has been increased to over 18 GHz. Simultaneous repetition rate multiplication of pulses has been performed at two different center wavelengths obtained from a pair of DFB laser diodes. The scheme can also be generalized to multiply multi-wavelength laser pulses over a wide spectral range.
By taking advantage of the relatively broad transmission peaks, our approach shows a large tolerance of 1.2 GHz to the variation of the input repetition rate. In addition, as inherited from the polarization independent operation of the LMF, the approach is polarization insensitive and the multiplication works well for both non-polarized and polarized laser sources, including mode-locked fiber ring lasers and gain-switched DFB laser diodes.
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