1. Introduction

Fiber lasers have attractive characteristics, including a narrow linewidth and a wide tunability for various applications such as metrology, sensing, and spectroscopy. In addition, fiber lasers make it easy to obtain a mode-locked femto-second pulse [1,2]. Especially, an ultra-short pulse fiber lasers can be used as an optical frequency comb generator (OFCG) [3] for absolute frequency measurement in the optics communication region. Previous reports have demonstrated a multi-wavelength source (MWS) with a microwave frequency interval based on modulation devices in the fiber loop by injection seeding [4,5]. To date, however, none of these demonstrations has utilized mode-locked pulse operation. The standard method for producing the MWS based on a fiber loop is to use an electro-optic modulator (EOM) [4,5] or an acousto-optic modulator (AOM) [6–10] in the optical fiber ring cavity. Although the output characteristic of the MWS is similar to that of a mode-locked fiber laser, which is a series of equally spaced modes in the frequency domain, the relative phase between the modes of this MWS are not coherent to each other. Although a passively mode-locked fiber laser makes it easy to operate a mode-locked pulse laser, the repetition rate of the passively mode-locked fiber laser is limited by the cavity length, and the actively mode-locked fiber based on the modulator devices never has been controlled the carrier envelope offset of the pulses. In order to apply to absolute frequency measurement, we need an actively mode-locked pulse laser with a high repetition rate and control of the carrier envelope offset of the pulses.

In this paper, we propose an OFCG based on an AOM operated in an actively mode-locked pulse mode, utilizing the injection-seeding technique (IST). The repetition rate of our OFCG depends on the driving frequency of the AOM, and the phases of the spectral components are coherent. Control of the carrier envelope offset of the pulses was achieved via the IST. Our OFCG can produce strong comb lines of over 1.8 THz, spaced by 150 MHz, within a -60 dBm power level. The frequency of the OFCG corresponds exactly to the deriving frequency of the AOM referred on the frequency of the seeding laser.

2. Experimental setup

Figure 1 shows a schematic of our experimental setup for the optical frequency comb generator based on the fiber ring cavity, using an AOM and the IST. The OFCG consists of a seeding laser, an AOM, a polarizer, an isolator, a polarization controller, a coupler, and erbium-doped fiber (EDF) of 1-m length forward pumped by a pump LD of 980 nm providing 300 mW of maximum power. The EDF with a mode field diameter of 6.5 µm has an unpumped absorption coefficient of 110 dB/m at 1530 nm and an estimated dispersion coefficient of -9 ps/nm-km at 1550 nm. The polarizer was used to guarantee a linearly polarized state and unidirectional operation with fiber coupled ports. The optical isolator was introduced to block the backward ASE generated in the EDF and to ensure unidirectional operation. A polarization controller was used not only to control the polarization state of the propagating light, but also to control the output lasing wavelength of the fiber laser. The free spectral range of the optical fiber cavity is a subharmonic of the modulation frequency produced by the AOM, and the frequency-shifted lights are again built up as a modulation of the isolated ECLD injected seeding. The AOM had a maximum diffraction efficiency of 65 % at 1550 nm, a carrier frequency of 150 MHz, and a rise time of 70 ns. The external injection source was used as the external cavity diode laser (Agilent 8164B ECLD) with a linewidth of 100 kHz, which can be operated in the lasing bandwidth of a fiber ring laser. The total cavity length of the loop had an optical pass length of 13.7 m, corresponding to cavity mode spacing of 15 MHz. In order to optimize the OFCG, the ratio of the injection-seeding input power and the optical fiber cavity output power was controlled by a tunable coupler, which was adjustable from 0 to 90 %. The output of the OFCG was simultaneously monitored on the optical spectrum and the electrical spectrum. In order to measure the electric spectrum, a photo detector with an 8-GHz bandwidth was used as an electro-optical converter. We obtained the beat signals between the generated optical frequency comb modes using an RF spectrum analyzer.

 

Fig. 1. Configuration of the experimental setup for a fiber optical frequency comb generator. The external cavity laser diode (ECLD) source was used for the injection seeding.

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3. Experimental results and discussions

Figure 2 shows the amplified spontaneous emission (ASE) spectrum from the high gain EDF when pump power of 200 mW was launched to the EDF through the WDM. While the ASE of the typical EDF generated at 1530 nm regions, the ASE peak of the high gain EDF appears the near 1560 nm region. This is due to the high absorption of the high-gain EDF to deplete the pump power. The pump depletion causes the re-absorption of shorter wavelengths in the 1530 nm region, and the power of the EDF is coupled to higher wavelengths.

 

Fig. 2. The spectrum of the forward amplified spontaneous emission (ASE) for an erbium doped fiber of 1-m length pumped at 200 mW. The ASE spectrum has a peak at 1560 nm instead of the usual one at 1530 nm due to depletion of pump power.

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Fig. 3. The variation of comb spectra on the RF spectrum in term of the different injection wavelengths from the externally seeded laser. The upper right inset is the lasing spectrum injected by a seed laser with a wavelength of 1565 nm.

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We first consider the output spectrum operated as the MWS before the mode-locked pulse. Figure 3 shows the formation process of the RF spectra of the MWS with 150 MHz spacing, according to the wavelength of the seeding laser. We can see that the frequency modes were strongly generated when the wavelength of the seeding laser was located within the lasing bandwidths of the fiber ring laser, as shown in the upper right inset in Fig. 3. Although the wavelength of the seeding laser was not located within the lasing bandwidths, some frequency modes were weakly generated with the seeding laser of 1555 nm. In other words, the wavelength of the seeding laser to produce effective multi-wavelength modes has to be at minimum a -60 dB power level of the lasing spectrum [11]. We verified that generation of strong multi-wavelength modes in the broad range could be achieved via injection-seeding from 1560 nm to 1567 nm into the fiber ring cavity.

Next, we consider operation of the MWS as a mode-locked pulse, as shown in Fig. 4. The configuration of our OFCG has an active mode locking scheme based on the frequency shifted feedback method [12,13] in terms of the coherent superposition of frequency shifted waves. Figure 4 shows the optical spectrum of the mode-locked pulse centered at 1562 nm, which has an approximate spectral width of 4 nm at -3-dB bandwidth. The pulse spectrum exhibits Kelly sidebands that are typically found in the output of a soliton laser [14]. To operate the mode-locked pulse, a pump power of 200 mW was launched into the EDF when the seed laser of 1565 nm was injected into the fiber ring cavity. Then, we could obtain the mode-locked pulse when the polarization state in the fiber laser cavity was consistent with the polarization axis of the linear polarizer by using the polarization controller. In particular, mode-locking could also be achieved by using other seed laser wavelengths near 1565 nm. The insert in Fig. 4 shows the optical pulse train monitored with a 100 MHz sampling oscilloscope and photo detector in the time domain when mode-locking was achieved. Additionally, we also observed the mode-locked pulse operation with a driving frequency of 80 MHz to the AOM in the same configuration. From this result, we can say that the modulation frequency was not dominant for the mode-locked condition. Even if higher modulation of over a few GHz were applied to the ring cavity loop, an OFCG with pulsed mode operation may be realized.

As mentioned above, when the wavelength of the seed laser is in the lasing region of a mode-locked fiber laser, strong comb lines are displayed on an RF analyzer with 150 MHz spacing, as shown in Fig. 5. A modulation frequency of 150 MHz was applied to the AOM device in the OFCG, and a seed laser of 1565 nm was injected into the fiber ring cavity. The entire comb spectra could not be observed over 10 GHz due to the bandwidth-limited 8 GHz of the photo receiver. We are convinced that optical frequency comb spectra of at least 1.8 THz can actually be generated, considering that shot noise limited power of mode locked output spectral width (~15 nm) was set to a -60 dBm power level. When the seed laser was not injected into the ring cavity, comb spacing lines on the RF spectrum appeared with cavity mode spacing of 15 MHz, corresponding to the optical path length of the fiber ring cavity.

Since the external cavity laser diode (ECLD) is injected into the fiber ring cavity, the component shifted 150 MHz by the AOM are again built up in the cavity after one round trip, and the repeated process creates the strong comb spectra through the EDFA. Because the frequency reference of the optical frequency comb is the frequency of the seed laser, the frequency of the comb changes according to the frequency of the seed laser. This means that we can control the carrier envelope offset of the mode-locked pulses. If the seed laser is a frequency stabilized laser into the transition line of an atom or a molecular, we will be able to accurately measure the arbitrary optical frequency using the frequency of the seed laser. The beat notes (comb spacing) between two adjacent modes exactly matched with the driving frequency of the AOM. This means that we can control the repetition rate of the mode-locked pulses. We obtained optimized comb spectra by adjusting the polarization controller and the coupling ratio of the tunable coupler. As the coupling ratio was tuned to 80 % for re-injection into the fiber ring loop as a function of the fiber cavity gain/loss, optimized strong comb lines were generated within the bandwidth of the photo detector. In particular, the linewidth between adjacent modes was measured to be less than 3 Hz. In this respect, the relative frequency stability can be estimated to be 10^-9 considering the frequency noise of the AOM driver has an order of ~Hz.

 

Fig. 4. Mode locked optical spectrum centered at 1562 nm with effective spectral width of 15 nm for comb generation from a fiber ring laser. The launched power of the pumping LD is 250 mW. The insert shows the pulse train measured by a 100 MHz sampling oscilloscope in the time domain.

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Fig. 5. The output spectra of comb generator with periodic frequency spacing when applied to the acousto-optic device by the modulation frequency of 150 MHz with mode-locked pulse operation. The wavelength and power of seed laser was 1565 nm and 1.6 mW, respectively.

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4. Conclusion

We have demonstrated an optical frequency comb generator (OFCG) with actively mode-locked fiber ring laser, using the AOM as a frequency shifter. We controlled the carrier envelope offset of the mode-locked pulses by using the injection-seeding technique and controlled the repetition rate by using the AOM. The spectrum bandwidth of the fiber OFCG is expected to generate over 1.8 THz from the shot noise limited power of a mode locked output spectral width of 15 nm. This result has the widest bandwidth ever reported for a fiber comb by using an AOM device. Because the harmonic combs were in phase, beat components between the modes could operate coherently with each other. To improve the stability of the comb mode and the seeding laser, further experiments are underway that use an acetylene stabilized laser with simultaneous synchronization of the AOM device with a hydrogen maser.