Two-way fiber-wireless convergence systems based on two-stage injection-locked VCSELs transmitter and optical interleaver

Chung-Yi Li; Hai-Han Lu; Cheng-Ling Ying; Jun-Ren Zheng; Che-Yu Lin; Zhi-Wei Wan

doi:10.1364/OE.23.005244

1. Introduction

With the rapid development of lightwave and radio frequency (RF) wireless transmission technologies, the increasing requirements raise the demands for high-speed and high-volume applications; not only for the fiber-based distribute network, but also for the RF wireless-based in-house one. A network that can provide both wired and wireless communications concurrently has been required for various applications [1–5]. A digitized fiber-wireless system employing sigma delta modulation was demonstrated previously [1]. However, sophisticated high-bandwidth analog-to-digital and digital-to-analog converters are required. And further, a fiber-wireless transmission system of polarization division multiplexing (PDM)-multiple input multiple output (MIMO)-orthogonal frequency division multiplexing (OFDM) at 100 GHz frequency was presented previously [2]. Nevertheless, such a fiber-wireless transmission system is not practical and flexible due to only one-way transmission. In addition, full-duplex radio-over-fiber (RoF) transmission systems were illustrated formerly [3–5]. Nonetheless, the intensity modulation (IM) up-link optical signal with large optical modulation index will induce performance degradation on the down-link one. To overcome the limitation, phase modulator can be applied in full-duplex RoF transmission systems for up-link transmission thanks to the constant intensity of phase modulation (PM) up-link optical signal. In this paper, a two-way fiber-wireless convergence system based on a two-stage injection-locked 1.55-μm vertical-cavity surface-emitting lasers (VCSELs) transmitter and an optical interleaver (IL) to deliver intensity-modulated and phase-remodulated millimeter-wave (MMW) data signals over a 40-km single-mode fiber (SMF) and a 5-m RF wireless transport is proposed and experimentally demonstrated. For up-link transmission, a PM-to-IM converter employing injection-locked VCSEL scheme is utilized to transform the phase-modulated optical signal into the intensity-modulated one [6, 7]. It is not necessary to employ a sophisticated delay interferometer (DI) to make a PM-to-IM conversion [8]. In recent years, the VCSEL technology has advanced enough that VCSEL can be designed to operate in the 1.55-μm wavelength window and in a single transverse mode for high-bandwidth operation. The new generation of VCSEL scheme is optimized for modulation bandwidth to few tens of GHz [9, 10]. One such promising VCSEL scheme is the two-stage injection-locked VCSELs. It breaks the bandwidth bottleneck of traditional laser diode (LD) and opens a promising approach for high-bandwidth operation [11]. One-stage injection-locked technique has been verified as a very effective way to enhance the frequency response of VCSEL [12]. It is expected to have good transmission performances in two-way fiber-wireless convergence systems. Two-stage injection-locked technique, which can further enhance the frequency response of VCSEL, is thereby expected to have excellent transmission performances in ones. Furthermore, an optical IL is deployed in two-way fiber-wireless convergence systems to separate off the optical carrier and the optical sidebands. The optical IL has two outputs, one of the outputs has the optical signal only with the optical sidebands (without optical carrier), and another output has the optical signal only with the optical carrier (without optical sidebands) [13]. The optical sidebands are employed for down-link transmission and the optical carrier is employed for up-link transmission. Wavelength reuse is popular and widely used in two-way lightwave transmission systems due to its economic and convenient installation characteristics [14]. By replacing wavelength-selected LD for up-link transmission, the network service providers can set up the optical network flexibly. To the best of our knowledge, it is the first time to employ two-stage injection-locked 1.55-μm VCSELs transmitter and optical IL in a two-way fiber-wireless convergence system. The downstream light is successfully phase-remodulated with MMW data signal for up-link transmission. The performances of downstream and upstream MMW data signals are measured and analyzed by parameters such as bit error rate (BER) and eye diagram. Through a serious investigation in two-way fiber-wireless convergence systems, BER and eye diagram perform brilliantly over a 40-km SMF and a 5-m RF wireless transport. Such a two-way fiber-wireless convergence system reveals a prominent alternative not only to present its advancement in integration of distribute fiber and in-house network, but also to offer the advantages of communication channels for higher bandwidth and data rates.

2. Experimental setup

The architecture of the proposed two-way fiber-wireless convergence systems based on a two-stage injection-locked 1.55-μm VCSELs transmitter and an optical IL is shown in Fig. 1. The two-stage injection-locked 1.55-μm VCSELs transmitter is directly modulated by a 10Gbps/30GHz MMW data signal. The modulated light is amplified by an erbium-doped filter amplifier (EDFA), attenuated by a variable optical attenuator (VOA), transmitted through a 40-km SMF link (with an attenuation of 0.24 dB/km, a dispersion coefficient of 17 ps/nm/km, and an effective area of 80µm²), and then reached to an optical IL. Since the VCSELs transmitter is modulated by a 10Gbps/30GHz MMW data signal, yet the generated optical sidebands remains coherent with each other and the channel spacing between the adjacent sidebands is fixed at 30 GHz. The −1 and + 1 sidebands are obtained from the optical IL output only with the optical sidebands. And thus the 10Gbps/30GHz MMW data signal can be optically promoted to the 10Gbps/60GHz one. The 10Gbps/60GHz MMW data signal is then detected by a 60-GHz photodiode (PD), boosted by a 60-GHz power amplifier (PA), and wirelessly transmitted by a 60-GHz horn antenna (HA). The 60-GHz PD has a detection wavelength range of 860-1630 nm and a responsivity of 0.7 mA/mW at 1550 nm. Over a 5-m RF wireless transmission, the 10Gbps/60GHz MMW data signal is received by a 60-GHz HA, amplified by a 60-GHz low-noise amplifier (LNA) with a noise figure and a small signal gain of around 4.2 and 13 dB, down-converted by a 60-GHz local oscillator (LO) and mixer, data recovered by a data recovery scheme, and fed into a BER tester (BERT) for BER performance analysis.

Fig. 1 The architecture of the proposed two-way fiber-wireless convergence systems based on a two-stage injection-locked 1.55-μm VCSELs transmitter and an optical IL.

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For up-link transmission, the optical IL output only with the optical carrier is launched into another phase modulator for phase-remodulating MMW data signal. The 10Gbps/30GHz MMW data signal is fed into a phase modulator, amplified by an EDFA, attenuated by a VOA, and transmitted by another 40-km SMF link. Over a 40-km SMF transport, the optical signal is passed through a VCSEL-based PM-to-IM converter, detected by a 30-GHz PD, boosted by a 30-GHz PA, and wirelessly transmitted by a 30-GHz HA. The PM-to-IM converter consists of an optical circulator (OC) and a VCSEL. As the VCSEL is injection-locked, the upper sideband ( + 1 sideband) of the phase-modulated optical signal will be amplified, while the lower sideband (−1 sideband) of the phase-modulated optical signal remains unchanged [15]. This leads to an optical PM-to-IM conversion. The 30-GHz PD has a detection wavelength range of 850-1650 nm and a responsivity of 0.85 mA/mW at 1550 nm. Over a 5-m RF wireless transmission, the 10Gbps/30GHz MMW data signal is received by a 30-GHz HA, amplified by a 30-GHz LNA with a noise figure and a small signal gain of around 3.2 and 20 dB, down-converted by a 30-GHz LO and mixer, data recovered by a data recovery scheme, and fed into a BERT for BER performance evaluation. In traditional two-way fiber-wireless convergence systems, the upstream MMW data signal is received by a wireless receiver and then delivered over a fiber link [1, 3, 4]. Thereby, they have put wireless receivers in same sides. In such way, the expensive high-bandwidth wireless amplifiers (60-GHz PA, HA, and LNA) for up-link transmission increase the cost of systems. However, in our proposed two-way fiber-wireless convergence systems, the upstream MMW data signal is delivered over a fiber link and then received by a wireless receiver. Thereby, we have put wireless receivers in both sides. In this way, the middle-bandwidth wireless amplifiers (30-GHz PA, HA, and LNA) for up-link transmission are employed in systems. It is shown to be a cost-effective option since expensive high-bandwidth wireless amplifiers for up-link transmission are not employed in systems.

The configuration of the directly modulated two-stage injection-locked 1.55-μm VCSELs transmitter is presented in Fig. 2. A distributed feedback (DFB) LD, with an optical power of 40 mW and a central wavelength of 1550.46 nm (λ₀), is used as the master laser. A 10-Gbps data stream is mixed with a 30-GHz MMW carrier to generate the 10Gbps/30GHz amplitude shift keying (ASK) data signal. Such a 10Gbps/30GHz ASK data signal is directly fed into the VCSEL1 with a central wavelength of 1550.44 nm (λ₁). The optical output of the DFB LD is injected into the VCSEL1 via an OC1 and a polarization controller (PC1) at an injection ratio of about 10 dB. Meanwhile, the VCSEL2, with a central wavelength of 1550.47 nm (λ₂), is injection-locked by VCSEL1 via the OC2 and PC2 at a strong injection ratio of about 14 dB. The three-port OC is worth employing due to excellent optical characteristics, including low insertion loss (~0.7 dB), high isolation (>42 dB), and environment stability. The PC is utilized to match the master polarization to the VCSEL preferred polarization to firm up the locking stability. The frequency response of the two-stage injection-locked 1.55-μm VCSELs transmitter is measured at the port 3 of OC2 by using an optical network analyzer.

Fig. 2 The configuration of the two-stage injection-locked 1.55-μm VCSELs transmitter.

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3. Two-stage injection-locked VCSELs

For the free-running VCSEL1, given that the operation current is operated at 8.3 times the threshold current, the 3-dB bandwidth is around 10.1 GHz. This finding indicates that VCSEL is designed for efficient use in high-bandwidth operation. Both VCSELs (VCSEL1 and VCSEL2) have the same frequency response characteristics. The frequency responses of one-stage injection-locked 1.55-μm VCSEL and two-stage injection-locked 1.55-μm VCSELs are shown in Fig. 3. For one-stage injection-locked 1.55-μm VCSEL, the 3-dB bandwidth is 48 GHz. For two-stage injection-locked 1.55-μm VCSELs, the 3-dB bandwidth is increased up to 63 GHz as expected. This finding suggests that the two-stage injection-locked 1.55-μm VCSELs transmitter is strong enough for 10Gbps/60GHz MMW data signal transmission. To obtain high 3-dB bandwidth, the wavelengths of the injected lights should be carefully chosen to obtain the optimal enhancement in the frequency response of two-stage injection-locked VCSELs. For the first-stage injection locking, the wavelength of the master laser (DFB LD) should be slightly longer than that of the slave laser (VCSEL1) to obtain a flat frequency response; that is, positive wavelength detuning is employed to achieve the first-stage injection locking. For the second-stage injection locking, however, the wavelength of the master laser (one-stage injection-locked VCSEL1) should be slightly shorter than that of the slave laser (VCSEL2) to obtain a high-frequency resonance peak; that is, negative wavelength detuning is employed to achieve the second-stage injection locking [16].

Fig. 3 The frequency responses of one-stage injection-locked 1.55-μm VCSEL and two-stage injection-locked 1.55-μm VCSELs.

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Meanwhile, the second slave laser (VCSEL2) needs to be injection-locked at a strong injection [17]. The dynamics of injection locking can be described by the rate equations that couple the field amplitude, field phase, and carrier number of the slave laser:

\frac{d A (t)}{d t} = \frac{1}{2} g [N (t) - N_{t h}] A (t) - k A_{i n j} \cos ϕ (t)

\frac{d ϕ (t)}{d t} = \frac{α}{2} g [N (t) - N_{t h}] - k \frac{A_{i n j}}{A (t)} \sin ϕ (t) - 2 π Δ f

\frac{d N (t)}{d t} = J - k γ {}_{N}N (t) - {γ {}_{P}+ g [N (t) - N_{t h}]} A^{2} (t)

where A(t) is the field amplitude, A_inj is the field amplitude injected into the slave laser,

ϕ (t)

is the phase difference between the laser field of the slave laser and the master laser, N(t) is the carrier number, J is the injection current, g is the linear gain coefficient, N_th is the threshold carrier number,

γ {}_{P}

is the photon decay rate, k is the coupling coefficient,

α

is the linewidth enhancement factor,

γ {}_{N}

is the carrier decay rate, and

Δ f

is the lasing frequency difference between the master and the slave laser. By performing a small signal linear analysis of Eqs. (1)-(3), an equation for the resonance frequency (f_r) of the injection-locked laser can be derived:

f_{r}^{2} ≅ f_{0}^{2} + k^{2} {(\frac{A_{i n j}}{A_{0}})}^{2} \sin^{2} ϕ_{0}

where f₀ is the relaxation frequency of the free-running slave laser, k is the coupling coefficient,

\frac{A_{i n j}}{A_{0}}

is the injection ratio, and

ϕ_{0}

is the phase difference between the injection-locked slave laser and the master laser. As shown in Eq. (4), the resonance frequency of the injection-locked slave laser is proportional to the square of the injection ratio. Strong injection results in high resonance frequency.

4. Experimental results and discussions

The optical spectra of different optical signals at several interesting points in the optical path are presented in Figs. 4(a)-4(e) [insert (a) - (e) of Fig. 1]. For down-link transmission, the optical spectra measured at the optical IL input [insert (a) of Fig. 1] and output only with optical sidebands [insert (b) of Fig. 1] are presented in Figs. 4(a) and 4(b), respectively. Meanwhile, the optical spectrum measured at the optical IL output only with optical carrier [insert (c) of Fig. 1] is shown in Fig. 4(c). For up-link transmission, the optical spectrum before the VCSEL-based PM-to-IM converter is shown in Fig. 4(d) [insert (d) of Fig. 1]. An injection locking enhances the intensity of the upper sideband and produces the optical spectrum as shown in Fig. 4(e) [insert (e) of Fig. 1].

Figures 4 (a) - 4(e) The optical spectra of different optical signals at several interesting points in the optical path [insert (a) - (e) of Fig. 1].

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For down-link transmission, the measured BER curves of 10Gbps/60GHz MMW data signal for back-to-back (BTB) and over a 40-km SMF as well as a 5-m RF wireless transport scenarios are shown in Fig. 5(a). A power penalty of 3.8 dB is observed between BTB and 40-km SMF as well as 5-m RF wireless transport scenarios at BER of 10⁻⁹. Such a result can be attributed to the use of two-stage injection-locked 1.55-μm VCSELs scheme. The use of two-stage injection-locked 1.55-μm VCSELs scheme extends the 3-dB bandwidth of optical transmitter greatly, by which leading to system with lower second-order harmonic distortion to carrier ratio (HD₂/C) and third-order intermodulation distortion to carrier ratio (IMD₃/C). HD₂/C and IMD₃/C can be expressed as [18]:

H D_{2} / C \approx m^{2} \cdot {| F R (2 f_{1}) |}^{2} \cdot {(\frac{f_{1}}{f_{r}})}^{4}

I M D_{3} / C \approx m^{4} \cdot [{{(\frac{f_{1}}{f_{r}})}^{4} - \frac{f_{1}^{2}}{2 f_{r}^{2}}}^{2} + {(\frac{f_{1}}{f_{0}})}^{2} {\frac{1}{4 π f_{r} τ_{n}} - {(\frac{f_{1}}{f_{r}})}^{2} (2 π f_{0} τ_{p} + \frac{3}{4 π f_{r} τ_{n}} + \frac{3 ε S_{0}}{2 π f_{r} τ_{p}})}^{2}]

where m is the optical modulation index, FR(f) is the small-signal frequency response, f₁ is the modulation frequency, S₀ is the photon density, τ_n is the recombination lifetime of carriers, τ_p is the photon lifetime, and ε is the gain compression parameter with respect to photon density. It is clear from Eqs. (5) and (6) that both HD₂/C and IMD₃/C can become very small when f₁ << f_r. These HD₂/C and IMD₃/C decrements provide systems with better BER performance and result in the improvement of receiver sensitivity.

Fig. 5 (a) The measured BER curves of 10Gbps/60GHz MMW data signal for BTB and over a 40-km SMF as well as a 5-m RF wireless transport scenarios (down-link transmission). (b) The measured BER curves of 10Gbps/30GHz MMW data signal for BTB and over a 40-km SMF as well as a 5-m RF wireless transport scenarios (up-link transmission).

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For up-link transmission, the measured BER curves of 10Gbps/30GHz MMW data signal for BTB and over a 40-km SMF as well as a 5-m RF wireless transport scenarios are shown in Fig. 5(b). A power penalty of 3.2 dB is observed between BTB and 40-km SMF as well as 5-m RF wireless transport scenarios at BER of 10⁻⁹. The result is due to the use of PM scheme. Since PM scheme is deployed in systems, yet distortion induced by fiber dispersion and RF wireless fading effects will be suppressed automatically. Such a dispersion suppression behavior leads to BER performance improvement [19].

Figures 6(a) and 6(b) display the eye diagrams under the conditions of 40-km SMF and 5-m RF wireless transport scenario for down-link transmission [Fig. 6(a)] and 40-km SMF and 5-m RF wireless transport one for up-link transmission [Fig. 6(b)]. Clear eye diagrams are obtained in the scenarios wherein two-stage injection-locked 1.55-μm VCSELs scheme (down-link transmission) and PM scheme (up-link transmission) are employed to suppress the amplitude and phase fluctuations.

Fig. 6 The eye diagrams under the conditions of 40-km SMF and 5-m RF wireless transport scenario for (a) down-link transmission, (b) up-link transmission.

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5. Conclusions

A two-way fiber-wireless convergence system based on a two-stage injection-locked 1.55-μm VCSELs transmitter and an optical IL is proposed and experimentally demonstrated. To be the first one of employing two-stage injection-locked 1.55-μm VCSELs transmitter and optical IL in two-way fiber-wireless convergence systems, the downstream light is successfully phase-remodulated with MMW data signal for up-link transmission. Impressive transmission performances of BER and eye diagram are obtained over a 40-km SMF and a 5-m RF wireless transport. Such a two-way fiber-wireless convergence system reveals an outstanding one not only to present its advancement in integration of distribute fiber and in-house network, but also to provide the advantages of the communication links for higher bandwidth and data rates.

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Two-way fiber-wireless convergence systems based on two-stage injection-locked VCSELs transmitter and optical interleaver

Abstract

1. Introduction

2. Experimental setup

3. Two-stage injection-locked VCSELs

4. Experimental results and discussions

5. Conclusions

References and links

Cited By

Figures (6)

Equations (6)

Optics Express