OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 8 — Apr. 13, 2009
  • pp: 6584–6590
« Show journal navigation

Generation of high repetition rate femtosecond pulses from a CW laser by a time-lens loop

Yitang Dai and Chris Xu  »View Author Affiliations


Optics Express, Vol. 17, Issue 8, pp. 6584-6590 (2009)
http://dx.doi.org/10.1364/OE.17.006584


View Full Text Article

Acrobat PDF (387 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We demonstrate a novel method for femtosecond pulse generation based on a time-lens loop. Time division multiplexing in the loop is performed so that a high repetition rate can be achieved. Pulse width less than 500 fs is generated from a continuous wave (CW) laser without mode locking, and tunable repetition rate from 23 MHz to 400 MHz is demonstrated. Theoretical analysis shows that the repetition rate is ultimately limited by the in-loop interference. By using a 2×2 optical switch, such interference is further suppressed, and repetition rate as high as 1.1 GHz is demonstrated.

© 2009 Optical Society of America

1. Introduction

Femtosecond pulses at high repetition rates are required for a variety of applications, such as ultrafast sampling [1

1. A. Bartels, R. Cerna, C. Kistner, A. Thoma, F. Hudert, C. Janke, and T. Dekorsy, “Ultrafast time-domain spectroscopy based on high-speed asynchronous optical sampling,” Rev. Sci. Instrum. 78, 351071 (2007). [CrossRef]

] and frequency metrology [2

2. S. T. Cundiff, “Metrology: new generation of combs,” Nature , 450, 1175–1176 (2007). [CrossRef] [PubMed]

]. Passive mode-locking is the main approach to achieve fs-pulses. The repetition rate of passive mode-locked laser is limited by the cavity length. Although a compact cavity [3

3. A. Bartels, T. Dekorsy, and H. Kurz, “Femtosecond Ti:sapphire ring laser with a 2-GHz repetition rate and its application in time-resolved spectroscopy,” Opt. Lett. 24, 996–998 (1999). [CrossRef]

, 4

4. T. M. Fortier, A. Bartels, and S. A. Diddams, “Octave-spanning Ti:sapphire laser with a repetition rate >1 GHz for optical frequency measurements and comparisons,” Opt. Lett. 31, 1011–1013 (2006). [CrossRef] [PubMed]

] or a combination of active modulation and passive mode-locking [5

5. C. X. Yu, H. A. Haus, E. P. Ippen, W. S. Wong, and A. Sysoliatin, “Gigahertz-repetition-rate mode-locked fiber laser for continuum generation,” Opt. Lett. 25, 1418–1420 (2000). [CrossRef]

] can be used to achieve a high repetition rate, passive mode-locked lasers have limited tunability in their repetition rate without changing the optical components. In addition, the repetition rate is sensitive to environmental perturbations unless active feedback stabilization is incorporated for the cavity length. Femtosecond pulse generation based on time-lens pulse compression has been demonstrated to have much better repetition-rate tunability and stability [6

6. T. Khayim, M. Yamauchi, D. Kim, and T. Kobayashi, “Femtosecond optical pulse generation from a CW laser using an electrooptic phase modulator featuring lens modulation,” IEEE J. Quantum Electron. 35, 1412 (1999). [CrossRef]

, 7

7. J. V. Howe, J. H. Lee, and C. Xu, “Generation of 3.5 nJ femtosecond pulses from a continuous-wave laser without mode locking,” Opt. Lett. 32, 1408 (2007). [CrossRef] [PubMed]

]. A time lens refers to a device that imposes a quadratic phase in the time domain onto a pulse, in analogy to a spatial lens imposing a quadratic phase in space onto a spatial profile [8

8. B. H. Kolner and M. Nazarathy, “Temporal imaging with a time lens,” Opt. Lett. 14, 630 (1989). [CrossRef] [PubMed]

, 9

9. J. V. Howe and C. Xu, “Ultrafast optical signal processing based upon space-time dualities,” J. Lightwave Technol. 24, 2649 (2006). [CrossRef]

]. A time lens can broaden or compress a pulse in the time domain. In practice, a time lens can be approximated by an electro-optic modulator driven by a sinusoidal electrical waveform that is properly synchronized with the incoming pulse train. The repetition rate and the temporal width of the output pulses are entirely determined by the driving frequency and the modulation strength, respectively. Thus, pulse generation by the time lens has significant advantages in the flexibility and control of the pulse train, i.e., the pulse width and repetition rate can be tuned without changing any optical components.

Ultra-large modulation depth is required to achieve a pulse width under 500 fs. Earlier experiments in which kilowatts of RF power were applied to a bulk modulator achieved 16.25 GHz, 550 fs pulses at 514 nm, but the pulse quality is poor, and difficult to amplify [6

6. T. Khayim, M. Yamauchi, D. Kim, and T. Kobayashi, “Femtosecond optical pulse generation from a CW laser using an electrooptic phase modulator featuring lens modulation,” IEEE J. Quantum Electron. 35, 1412 (1999). [CrossRef]

]. More practical time-lens pulse generators have been limited to compressed pulse widths of around 2 ps. To get fs-pulses with low RF power (~ 1W), a loop containing only one time lens has been demonstrated to emulate many stacked lenses [7

7. J. V. Howe, J. H. Lee, and C. Xu, “Generation of 3.5 nJ femtosecond pulses from a continuous-wave laser without mode locking,” Opt. Lett. 32, 1408 (2007). [CrossRef] [PubMed]

]. In such a time-lens loop two 2×2 switches were used to control the injection and ejection of the pulses. The input pulse propagates multiple round-trips in the loops in order to obtain the required modulation depth. By effectively stacking the time lenses, 516 fs pulses at 1.55 μm were generated from a continuous-wave (CW) laser. However, the all-fiber loop, which contains a number of necessary optical components, is a few meters long. Since the minimum interval between adjacent pulses is the total time the pulse resides in the loop (i.e., round-trip time of the loop multiplied by the number of loops), the output pulse train has a low repetition rate of 3.18 MHz. Thus, in the demonstrated time-lens systems, there is a trade-off between the pulse width and the repetition rate. The shortest pulse is obtained at the expense of a low repetition rate.

2. System design

Figure 1(a) shows the proposed time-division multiplex time-lens loop. The femtosecond pulse generator consists of a seed source shown before point A, a time-lens loop shown between points A and C where chirped optical bandwidth is generated, followed by an amplification and compression stage beyond point C. The general operating principle is to allow pulses, which come from the seed source, to circulate the loop many times where they will acquire bandwidth for every pass through the time lens. After generating the desired bandwidth, pulses are ejected from the loop, amplified and then dechirped.

The seed source consists of a DFB laser at 1.55 μm. The 13 dBm CW output of the laser is pulse carved into 33-ps, ~ 10-GHz RZ-pulse train by a Mach-Zehnder modulator (MZM). The return-to-zero (RZ) pulses are amplified and pulse-picked one out of every M pulses by an intensity modulator. T is the period of the original RZ-pulse, T ~ 100 ps in our experiment. After filtering the ASE noise, pulses are injected into the time-lens loop. The power of the input pulse train is maintained at -0.8 dBm for different input repetition rate in our experiment. Inside the loop an erbium-doped waveguide amplifier (EDWA) is used to compensate for the loss. Note that one phase modulator is drawn for clarity, though there are actually two modulators inside the loop. Each modulator is driven at approximately 1.0 W of RF power by a ~ 10 GHz sinusoid. The total phase modulation is approximately 10π radians per pass. Obviously the round-trip-time of the loop has to be NT (N is an integer) in order to synchronize the time lens and the input pulse train.

Fig. 1. (a) Experiment setup of the time-division multiplexed time-lens loop based on a 3-dB coupler. EDFA: erbium-doped fiber amplifier; BPF: bandpass filter; PM: phase modulator; M: mirror; G: grating. (b) The principle of the multiplexing of the loop.

The principle of the time-division multiplexing of the loop is shown in Fig. 1(b). Each input pulse will occupy one of the N slots of the loop after injection. Because the loop length (NT) and the pulse period after pulse picker 1 (MT) are both integer numbers of T, pulse overlap will occur. Pulse overlap between the in-loop pulse and the newly-injected pulse occurs after the first input pulse traveled in the loop for a total time (NTx, where x is the number of loops traveled) that equals to an integer number (y) of periods of the pulse-picked pulse train (MTy), i.e. the first pulse overlaps the (y+1)th pulse. The equation that governs this overlapping condition is then

N×x=M×y
(1)

Since the first pulse is not overlapped by the pulses between the second and the yth pulse, x and y are co-prime (i.e., no common factor other than 1).

Fig. 2. (a) The calculated FROG trace of the 9-loop and 34-loop pulses. (b) The intensity profile of the output pulse.

Since each pulse injected into the loop will generate a group of pulses sequentially experiencing 0, 1, 2, 3, … loops, all the groups of pulses are then interleaved together (without interference based on the above analysis) at the output of the loop, point B in Fig. 1, forming a pulse train with repetition rate much higher than 1/MT due to the multiplexing. Note that the periodicity of such pulse group is still MT, so the pulse experiencing the desired loop number (9 in our design) in each group can then be easily picked by an intensity modulator, the pulse picker 2 in Fig. 1(a), which is driven by the pattern generator. At point C, the output pulse train with desired bandwidth and a repetition rate of 1/MT is then generated.

The high repetition rate and large tunability of the proposed time-lens loop is also indicated by Eq. (1). Without changing any optical component, N is a constant. M is then selected so that x can be large enough to ignore the pulse interference in the loop. In the simplest case where N is a prime number, we get y=N and x=M. We assume that xmin is the minimum value so that we can ignore the in-loop interference. For example, in our loop xmin is 25 if ±2% power fluctuation is the maximum interference allowed. Then the value of M can be, in theory, any integer lager than xmin except the multiples of N. Thus, the repetition rate, 1/MT, can be almost any value less than 1/xminT. The highest repetition rate, 1/xminT, is no longer constrained by the loop length and the number of loops, a remarkable flexibility that is not achievable before. The stability of the frequency and phase of the generated ultrashort pulses could also be predicted since the pulses are generated from a CW laser without mode-locking. High-quality coherence could be achieved by using a stabilized, narrow line width CW source.

3. Experiment result

After ejection from the loop the pulses were amplified to 11.6 dBm and compressed by a grating pair which gives approximately 1.36 ps2 of anomalous dispersion. Figure 3(b) shows the measured second-order interferometric autocorrelation trace with the calculated trace in the inset. The autocorrelation trace gives a pulse width of 688 fs. Taking into account the deconvolution factor calculated for this pulse shape gives a pulse width of 436 fs. Pulse trains with different repetition rates were obtained by tuning the value of M. The same amplification and dechirping were applied to the pulse trains at 23.40 MHz and 79.55 MHz, and the measured pulse widths are 457 fs and 459 fs, respectively.

Fig. 3. (a) The measured spectrum at point C in Fig. 1(a). The spectrum was taken at 0.2 nm resolution bandwidth. Insert: (left) an example of the measured output at point B; (right) the measured time-domain pulse shape at point C corresponding to the spectrum. (b) The measured interferometric autocorrelation trace of the dechirped pulse giving a 688 fs autocorrelation width with 436 fs deconvolved. Insert: calculated trace giving 673 fs autocorrelation width with 427 fs deconvolved.

Fig. 4. Experiment setup of the time-division multiplexed time-lens loop based on a 2×2 switch.
Fig. 5. (a) The measured spectrum and the time-domain pulse shape (insert) at point B in Fig. 4(a). The spectrum was taken at 0.2 nm resolution bandwidth. (b) The measured interferometric autocorrelation trace of the dechirped pulse giving a 742 fs autocorrelation width and 471 fs deconvolved. Insert: calculated trace giving 673 fs autocorrelation width with 427 fs deconvolved.

An alternative, perhaps more flexible, approach is to construct the time lens loop by inserting the time dependent loss (the 2×2 switch or a simple MZM) before the 3-dB coupler inside the loop in Fig. 1(a). The advantage of such a design is that the repetition rate can be tuned based on Eq. (1) without changing the output pulse width while the highest repetition rate of 1.1 GHz can still be achieved. In order to overcome the insertion loss of the 3 dB coupler and the switch, however, a higher gain amplifier inside the loop will be required.

4. Conclusion

In summary, we demonstrated a novel time-lens loop to generate femtosecond pulses with high repetition rate. A 3-dB-coupler-based time-lens loop was proposed to time-division multiplex the use of the time lens so that high repetition rate can be achieved. Femtosecond pulses with pulse width less than 500 fs and repetition rate from about 23 MHz to 400 MHz were obtained from a CW laser without mode-locking. A theoretical analysis was performed and showed that the maximum repetition rate was limited by the pulse interference within the loop. In order to decrease such interference, a 2×2 switch was used to replace the 3-dB coupler, and femtosecond pulse with 1.1 GHz repetition rate was demonstrated. Our system is compact, robust, and all fiber. Our technique can be extended to 1.06 μm and 1.3 μm where spectral bandwidth is even easier to generate due to the high efficiency of the phase modulator.

References and links

1.

A. Bartels, R. Cerna, C. Kistner, A. Thoma, F. Hudert, C. Janke, and T. Dekorsy, “Ultrafast time-domain spectroscopy based on high-speed asynchronous optical sampling,” Rev. Sci. Instrum. 78, 351071 (2007). [CrossRef]

2.

S. T. Cundiff, “Metrology: new generation of combs,” Nature , 450, 1175–1176 (2007). [CrossRef] [PubMed]

3.

A. Bartels, T. Dekorsy, and H. Kurz, “Femtosecond Ti:sapphire ring laser with a 2-GHz repetition rate and its application in time-resolved spectroscopy,” Opt. Lett. 24, 996–998 (1999). [CrossRef]

4.

T. M. Fortier, A. Bartels, and S. A. Diddams, “Octave-spanning Ti:sapphire laser with a repetition rate >1 GHz for optical frequency measurements and comparisons,” Opt. Lett. 31, 1011–1013 (2006). [CrossRef] [PubMed]

5.

C. X. Yu, H. A. Haus, E. P. Ippen, W. S. Wong, and A. Sysoliatin, “Gigahertz-repetition-rate mode-locked fiber laser for continuum generation,” Opt. Lett. 25, 1418–1420 (2000). [CrossRef]

6.

T. Khayim, M. Yamauchi, D. Kim, and T. Kobayashi, “Femtosecond optical pulse generation from a CW laser using an electrooptic phase modulator featuring lens modulation,” IEEE J. Quantum Electron. 35, 1412 (1999). [CrossRef]

7.

J. V. Howe, J. H. Lee, and C. Xu, “Generation of 3.5 nJ femtosecond pulses from a continuous-wave laser without mode locking,” Opt. Lett. 32, 1408 (2007). [CrossRef] [PubMed]

8.

B. H. Kolner and M. Nazarathy, “Temporal imaging with a time lens,” Opt. Lett. 14, 630 (1989). [CrossRef] [PubMed]

9.

J. V. Howe and C. Xu, “Ultrafast optical signal processing based upon space-time dualities,” J. Lightwave Technol. 24, 2649 (2006). [CrossRef]

OCIS Codes
(060.2380) Fiber optics and optical communications : Fiber optics sources and detectors
(320.5520) Ultrafast optics : Pulse compression
(320.7090) Ultrafast optics : Ultrafast lasers

ToC Category:
Ultrafast Optics

History
Original Manuscript: February 20, 2009
Revised Manuscript: March 30, 2009
Manuscript Accepted: March 31, 2009
Published: April 6, 2009

Citation
Yitang Dai and Chris Xu, "Generation of high repetition rate femtosecond pulses from a CW laser by a time-lens loop," Opt. Express 17, 6584-6590 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-8-6584


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. A. Bartels, R. Cerna, C. Kistner, A. Thoma, F. Hudert, C. Janke, and T. Dekorsy, "Ultrafast time-domain spectroscopy based on high-speed asynchronous optical sampling," Rev. Sci. Instrum. 78, 351071 (2007). [CrossRef]
  2. S. T. Cundiff, "Metrology: new generation of combs," Nature,  450, 1175-1176 (2007). [CrossRef] [PubMed]
  3. A. Bartels, T. Dekorsy, and H. Kurz, "Femtosecond Ti:sapphire ring laser with a 2-GHz repetition rate and its application in time-resolved spectroscopy," Opt. Lett. 24, 996-998 (1999). [CrossRef]
  4. T. M. Fortier, A. Bartels, and S. A. Diddams, "Octave-spanning Ti:sapphire laser with a repetition rate >1 GHz for optical frequency measurements and comparisons," Opt. Lett. 31, 1011-1013 (2006). [CrossRef] [PubMed]
  5. C. X. Yu, H. A. Haus, E. P. Ippen, W. S. Wong, and A. Sysoliatin, "Gigahertz-repetition-rate mode-locked fiber laser for continuum generation," Opt. Lett. 25, 1418-1420 (2000). [CrossRef]
  6. T. Khayim, M. Yamauchi, D. Kim, and T. Kobayashi, "Femtosecond optical pulse generation from a CW laser using an electrooptic phase modulator featuring lens modulation," IEEE J. Quantum Electron. 35, 1412 (1999). [CrossRef]
  7. J. V. Howe, J. H. Lee, and C. Xu, "Generation of 3.5 nJ femtosecond pulses from a continuous-wave laser without mode locking," Opt. Lett. 32, 1408 (2007). [CrossRef] [PubMed]
  8. B. H. Kolner and M. Nazarathy, "Temporal imaging with a time lens," Opt. Lett. 14, 630 (1989). [CrossRef] [PubMed]
  9. J. V. Howe and C. Xu, "Ultrafast optical signal processing based upon space-time dualities," J. Lightwave Technol. 24, 2649 (2006). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited