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We demonstrate single-mode fiber transmission distance enhancement up to 120 km of a directly-modulated injection-locked VCSEL modulated by a 10Gb/s NRZ signal. Injection locking induced data pattern inversion of the VCSEL causes adjustable chirp, which greatly extends reach. Both experiments and simulations are shown to explain this phenomenon.
©2009 Optical Society of America
1. Introduction
Directly modulated vertical-cavity surface-emitting laser (VCSELs) are attractive candidates as cost-effective optical transmitters for metro-area networks (MANs), local area networks (LANs) and high-speed Ethernet applications. A directly modulated VCSEL is desirable because it is compact, cost-effective and consumes a small amount of power. However, VCSELs have an intrinsic problem of frequency chirp, which significantly limits their transmission distance through standard single-mode fibers [1]. Hence, in addition to the modulation bandwidth, chirp is a very important figure-of-merit to gauge transmitter performance. It has been demonstrated that optical injection locking (OIL) can greatly enhance the modulation bandwidth of a directly-modulated laser. Previously, we reported a factor of 10 increase of modulation bandwidth and resonance frequency in an OIL-VCSEL [2,3].
In this paper, we present a completely new approach to achieve adjustable chirp in a directly modulated VCSEL using optical injection locking. We show that OIL can be used to change the polarity of the chirp of a VCSEL, from a positive chirp for a standard VCSEL to a negative chirp. The resulting negative chirp compensates the chromatic dispersion of standard single-mode fiber (SSMF) at 1550 nm, and increases the 10 Gbps transmission distance up to ~120 km for a single mode VCSEL, more than 10X compared to a free-running VCSEL. Theoretical simulations are presented to explain the OIL conditions with which the negative chirp can be achieved. Excellent agreement is obtained between experimental and theoretical results. This all-optical approach will be widely applicable to various modulation formats, bandwidths, and fibers and can even be applied as a post-deployment enhancement for VCSEL-based transmission systems.
2. Background
The frequency chirp in a VCSEL arises from the intrinsic dependence of refractive index in the laser active medium on current modulation. This leads to a frequency transient (transient chirp) and shift (adiabatic chirp) in the optical pulses emitted by directly modulated lasers (DML) [1]. As the drive current increases, the carrier density increases (rather than being clamped at a fixed value due to a relatively large spontaneous emission factor), the material gain and laser output power all increase. Based on Kramers-Kronig (K-K) relationship, this results in a decrease in the real part of refractive index, which leads to an increase in laser frequency. Hence, a positive transient chirp is observed on the rising edge of an optical pulse and negative on the falling one. The positive transient chirp and, to a smaller extent, the adiabatic chirp pull optical pulses apart when they travel in a standard single-mode fiber (SSMF), where higher frequency part of an optical pulse travel faster than the lower frequency one. Over a certain distance, the intensity of one pulse spreads over one bit period and causes detection errors. Using optical injection locking, the adiabatic chirp of a DM slave laser was reduced to a negligible value [4,5]. This is expected because the frequency of a DM laser should adiabatically reach its frequency under continuous wave (CW) operation, which is, for an OIL-laser, locked to that of the CW master laser. However, at high bit rates, it is the transient chirp that impacts the transmission distance the most.
A pre-chirp scheme is one of the most effective approaches to increase the transmission distance where the transmitter pulses are pre-adjusted before launched into the fiber link. This can be done by shaping the current pulses of electronic drivers or by various coding techniques [6]. However, these measures lack flexibility as they are fixed for a given modulation bandwidth and format, fiber type and distance. An optical approach using passive filters was demonstrated [7], but with limited control of tuning, integration potential and modulation bandwidth.
Since the K-K relation is fundamental, the most effective mechanism to reduce transient chirp of a DM laser is to invert the optical signal pattern (i.e. relative to the modulated current pattern). As such, a positive transient chirp can be changed to a negative one, which can greatly enhance transmission distance. In the following, we show that an OIL VCSEL can be conditioned to induce data inversion.
3. Physics of Adjustable Chirp of an OIL VCSEL
When a VCSEL is injection-locked, its threshold carrier density is reduced [8]. Based on K-K relation, the real part of refractive index of the gain medium is increased, which shifts the VCSEL cavity mode to a longer wavelength. The locked VCSEL cavity behaves like a gain-clamped narrow-band amplifier [8]. In this situation, the spectra show two peaks: a strong laser emission at the master wavelength (λ master) and a substantially smaller peak from the red-shifted slave cavity (λ cavity) [8]. The wavelength and amplitude of this side peak depends on the detuning Δλ, i.e. the master wavelength minus the free-running slave wavelength. Figure 1(a) shows the reflection spectra of an OIL-VCSEL under the same injection ratio but at various detuning values. At small Δλ (curve i), the amplitude of the side peak is positive, which manifests into a greatly enhanced modulation resonance frequency (equal to c/λ master-c/λ cavity, where c is the speed of light) [8].
What has never been measured is the fact that, with a large Δλ, the amplitude of the side peak goes negative, as shown in curves iii to vi of Fig. 1(a). Hence, by adjusting the detuning value, the gain-clamped amplifier can be tuned into a narrow-band absorber. The modulation response of an OIL-VCSEL is directly proportional to the amplitude change of the gain-clamped amplifier/absorber (the side peak). Figure 1(a) was measured using an Er-doped fiber amplifier (EDFA) as a broadband source, which was combined with the master laser and coupled into a VCSEL. The reflection spectrum was measured when the VCSEL was injection locked by the master laser at various detuning values.
Next, let us review the behavior of a VCSEL biased below threshold. Figure 1(b) shows the reflection spectrum of a VCSEL at various bias levels measured by illuminating it with an EDFA. At very low current, the Fabry-Perot mode is a relatively shallow dip due to the absorption in the active region. As current increases, the peak becomes deeper and narrower, as absorption decreases, and its wavelength blue shifts, as dictated by the K-K relation. A minimum reflection power occurs at the cavity transparency (I=0.32 mA in Fig. 1(b)) where the material gain (γ) is equal to the material loss (α i), so that the active cavity is equivalent to a lossless cold cavity without any gain medium. Biasing above cavity transparency, the device is in gain regime (γ>α i); whereas biasing below cavity transparency, the device is in loss regime (γ<α i). An electrical-to-optical transfer function can be extracted from Fig. 1(b) by plotting reflectivity at a fixed wavelength vs. the bias current. This is shown in Fig. 1(c). For a DC bias current greater than ~0.3 mA, the optical output follows the current modulation pattern. Whereas, if the bias is less than ~0.3 mA, the optical output shows an inverted pattern of the current modulation, and hence data inversion.
Using Fig. 1 (b) to explain the cavity peaks in Fig. 1(a), we show that the optical output of an OIL-VCSEL can follow or invert the signal pattern when the cavity is conditioned to follow a positive or negative slope on the transfer function, respectively. When the data pattern is inverted with respect to the current pattern, rising edge becomes falling edge and vise versa. However, the sign of chirp transient stays unchanged with the current pattern, as the frequency deviation follows the carrier density change that is determined by the current modulation. As a result, a negative chirp can be obtained through data pattern inversion.
Detuning dependence in Fig. 1(a) shows that the gain level of the gain-clamped amplifier is reduced as the master laser wavelength is red-tuned (toward longer wavelength). So the amplifer/absorber regime of an OIL VCSEL can be selected by changing the detuning, thus controlling the phenomenon of data inversion. Similarly, based on the above explanation and the gain-clamped amplifier model in [8], the amplifer/absorber regime can be selected with injection power ratio or bias current (not shown here).
Fig. 1. Measured reflection spectra of an OIL VCSEL. (a) Reflection spectra of an OIL VCSEL at various detuning values. 10 dB/div. (b) Reflection spectra of a VCSEL amplifier at various bias current levels below lasing threshold. 5 dB/div. (c) Electrical-to-optical transfer function for a VCSEL below threshold, as extracted from (b) at a fixed wavelength. The optical output shows an inverted pattern of the current modulation, and hence data inversion, for bias less than ~0.3 mA.
Figure 2 shows detuning-controlled data inversion with a OIL VCSEL modulated with a 2.5-Gb/s square pulse train at various detuning values. The waveform from a free-running VCSEL is shown as a reference. As expected from the VCSEL amplifier characteristics, the data pattern goes through a transition from a non-inverted to an inverted state as the wavelength detuning increases (master laser tuned to red) from 0 to 0.34 nm. The spikes appearing on the transition waveform (detuning=0.2 nm) are induced by the transient chirp associated with the rising and falling edge of the pulse.
Fig. 2. Experimentally measured waveforms showing detuning-controlled inversion phenomenon of a VCSEL when it is free-running as well as injection-locked at wavelength detuning of 0, 0.2 and 0.34 nm.
To understand this behavior, we established a hybrid model composed of both injection-locking rate equations and a simple amplifier structure [8]. The amplifier gain is modeled with 7 layers of QWs. The laser-structure-independent transient chirp is included as the derivative of the emission power, thus the carrier density change. The reflected intensity of the master laser is simulated as the output. Figure 3 shows the simulated modulation waveforms of the light output when the carrier density is modulated by the same waveform at three different detuning conditions. Excellent agreement is obtained with the experimental results.
Fig. 3. Simulated modulation output of an OIL VCSEL at various wavelength detuning values using a hybrid model combining OIL rate equations and a Fabry-Perot amplifier structure.
A negative chirp is indeed verified by measuring time-resolved frequency deviation. The waveform is measured using a digital sampling oscilloscope. The time-resolved chirp is obtained from ADVANTEST chirp analyzer. Figure 4 shows bit pattern intensity as well as the chirp waveform for both free-running (top) and OIL VCSEL tuned into inversion regime (bottom). A direct-modulated VCSEL possesses strong positive transient chirp with peak-to-peak value of 30 GHz. The OIL VCSEL, on the other hand, with the bits inverted illustrated by the dotted alignment lines has peak-to-peak transient chirp to less than 3 GHz.
In addition, the peak-to-peak negative chirp can be increased with the injection power ratio and longer optimal transmission distance [10]. This adjustability enables a network configuration to selectively transmit the signal to a destination on-the-fly by dynamically adjusting the negative chirp value.
Fig. 4. Measured intensity and chirp waveforms for both free-running and injection-locked VCSELs. The peak-to-peak transient chirp is greatly reduce by ~10X in the injection-locked condition.
4. Extended Reach of a Directly-modulated OIL VCSEL
The adjustable chirp discussed above results in an order of magnitude increase in single mode fiber transmission distance. The experimental apparatus includes a 1550-nm buried-tunnel-junction (BTJ) single-mode VCSEL [11]. The VCSEL has a threshold of 0.6 mA and maximum output power of ~1.5 mW biased at 10 mA. Emission is coupled into a single-mode lensed-fiber with efficiency ~70%. The master laser for injection locking is a FITEL continuous-wave DFB laser with maximum emission power of ~40 mW at 350 mA bias. OIL is performed by connecting the master laser, the VCSEL to port 1, and port 2 of an optical circulator, respectively. Port 3 of the circulator is the output for characterization. A 3-dB optical power splitter can be substituted for the circulator to lower cost with a tradeoff in higher master laser output power and negligible performance degradation [12]. A polarization controller is inserted between the circulator port 2 and the lensed-fiber to match the polarization of the master laser and the VCSEL. The data pattern is generated by a bit-error-rate test set (BERT) and directly modulates the VCSEL bias current. Transmission link is built by connecting fiber spools with various lengths. The BER performance is tested by modulating the VCSEL with a PRBS (215-1) at 10 Gb/s.
The experimental results are shown in Fig. 5, which plots the power penalty versus SSMF transmission distance when the VCSEL is directly modulated by 10Gb/s pseudo-random bit sequence (PRBS). Error-free (BER<10-9) power penalty referenced to free-running back-to-back as a function of transmission distance is measured for both a free-running VCSEL and an OIL VCSEL with negative chirp. An optimal transmission appears at a distance (25 km in this case) where the error-free receiver power is minimal due to compensation between chromatic dispersion of the fiber and the negative chirp of an OIL VCSEL. The transmission distance and dispersion tolerance are significantly enhanced. In a practical system, the transmission distances for various experimental conditions are typically compared at a fixed power penalty, for example, 4 dB here. For a DM VCSEL, the power penalty rapidly increases with transmission distance due to chirp, and the 4 dB-distance is limited to less than 10 km. However, under OIL conditions in this paper, the power penalty decreases with distance for the first 25 km to 0-dB power penalty, and subsequently increases at a slower rate with fiber distance. The 4 dB-distance is enhanced by one-order-of-magnitude to ~120 km. This behavior suggests the existence of negative chirp, which compensates the fiber dispersion to result in an optimal transmission distance with minimal power penalty. Eye diagrams in back-to-back, 25-km transmission and 100-km transmission conditions are also shown, which clearly demonstrates pulse compression due to dispersion compensation.
Fig. 5. Transmission measurements demonstrating distance enhancement of a direct-modulated OIL VCSEL with negative chirp at 10 Gb/s. Eye diagrams for OIL VCSEL at back-to-back (B2B), after 25-km and 100-km transmission are shown.
5. Conclusion
In summary, we demonstrate a novel approach to increase the transmission distance of a 10-Gb/s DM-VCSEL by more than 10 times to 120 km using OIL to control the sign and magnitude of chirp. We utilize the amplifier nature of an OIL-VCSEL to separate the intensity and frequency modulation of a DML. Therefore, the high-speed optical pulses from a direct-modulated OIL VCSEL can be pre-chirped, turning the chirp polarity from positive to negative through data pattern inversion by properly adjusting the wavelength detuning. Peak-to-peak chirp of an OIL VCSEL can be reduced by more than 10 times and controlled by the injection ratio. This feature makes an injection-locked VCSEL a strong candidate for future high-speed optical communication.
Acknowledgements
The authors thank Dr. Tingye Li (AT&T Bell labs, retired) for stimulating discussions and DARPA program N00014-08-1181 for funding.
References and links
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