Simultaneous MMW generation and up-conversion for WDM-ROF systems based on FP laser

Optics & Laser Technology 84 (2016) 94–101

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Simultaneous MMW generation and up-conversion for WDM-ROF systems based on FP laser Chan Zhang a,b, TiGang Ning a,b, Jing Li a,b,n, Chao Li a,b, Xueqing He a,b, Li Pei a,b a b

Key Laboratory of All Optical Network & Advanced Telecommunication Network of EMC, Beijing Jiaotong University, Beijing 100044, China Institute of Lightwave Technology, Beijing Jiaotong University, Beijing 100044, China

art ic l e i nf o

a b s t r a c t

Article history: Received 26 January 2016 Received in revised form 7 May 2016 Accepted 17 May 2016

A new wavelength division multiplexing radio-over-fiber (WDM-ROF) scheme based on Fabry-Perot (FP) laser is proposed and demonstrated for simultaneous millimeter-wave (MMW) generation and upconversion. The tunable optical comb generated by FP laser is served as a cost-effective WDM optical source in central station (CS) and it makes all-optical up-conversion process for all channels simple compared with using a DFB array. All modes from the FP laser are modulated simultaneously by a LiNbO3 Mach-Zehnder modulator (LN-MZM) then. We have systematically compared the performances of MMW generation and up-conversion using LN-MZM based on different modulation schemes. A reflective semiconductor optical amplifiers (RSOA) is used both for the downstream modulation of each channel and for the reduction of mode partition noise (MPN) induced from FP laser. In the scheme, the multiple optical carrier suppression (OCS) modulation shows the highest receiver sensitivity and smallest power penalty over long-distance delivery. In the numerical simulation, 7 WDM channels each carrying 2.5 Gb/s baseband signal have been up-converted to 60 GHz simultaneously with good performance over 25 km single mode fiber (SMF) transmission. & 2016 Elsevier Ltd. All rights reserved.

Keywords: WDM-ROF FP laser LN-MZM MMW Up-conversion

1. Introduction Wavelength division multiplexed passive optical networks (WDM-PON) have been considered as an attractive solution for an access network because of its large bandwidth, high security and protocol transparency [1–5]. However, they have not been widely deployed in real situations until recently because of high costs and low transmission rate. Radio-over-fiber (ROF), which combines the advantages of high efficiency, high compatibility and multi-service, has recently become a promising technique for future wireless communication [6–8]. In order to better exploit the enormous bandwidth of optical fiber and to further increase the system capacity with lower cost, WDM in combination with ROF (WDMROF) reveals significant potential for next generation broadband network [9–11]. A typical WDM-ROF system includes CS, optical fiber transmission, the remote node (RN) and remote BSs, signals are generated at the CS and then distributed to the remote base station (BS) by using optical fiber before being transmitted by antenna. For WDM-ROF systems, one key issue is to develop a compact and cost-effective multi-wavelength light source to n Corresponding author at: Key Laboratory of All Optical Network & Advanced Telecommunication Network of EMC, Beijing Jiaotong University, Beijing 100044, China. E-mail address: [email protected] (J. Li).

http://dx.doi.org/10.1016/j.optlastec.2016.05.010 0030-3992/& 2016 Elsevier Ltd. All rights reserved.

reduce the cost and consumption of the system. In most cases, mode-locked lasers, an array of distributed feedback laser (DFB) or a super-continuum light source are employed as the WDM optical source to provide multiple WDM channels [12–14]. However, the mode-locked lasers are very expensive and the overall cost increases when multiple independent lasers are used. Therefore, it is necessary to minimize the cost of the CS due to high atmospheric attenuation and complexity of the system. To build a number of multi-wavelength carriers in a CS for WDM-ROF system, the optical comb source is a promising candidate to serve as the WDM optical source [15,16]. An optical comb source with multiple comb lines can provide many WDM channels which can greatly increase the capacity and mobility of WDM-ROF system. Various methods have been reported to generate optical comb [17–20], but all these techniques require either expensive optical sources or complex configuration. As an alternative to solve these problems, a FP laser which has a wide spectrum spread with large output power and good frequency tunability has been studied. Very recently, FP laser has been used for signal processing, which is a new field for the application of FP laser [21]. Based on an FP cavity with practically feasible parameters, a photonic temporal integrator with an integration time window of 160 ns and an operation bandwidth of 180 GHz is achieved. On the other hand, with its multiple longitudinal modes characteristics and advantage of direct modulation, FP laser makes all-optical up-conversion process for all channels

C. Zhang et al. / Optics & Laser Technology 84 (2016) 94–101

simple compared with using a DFB array. Therefore, FP laser has been considered as a cost-effective WDM-ROF optical source in CS. In addition, MMW generation and all optical up-conversion is another key technique in WDM-ROF systems. A few schemes have been reported using external modulators based on different modulation schemes including double-sideband (DSB), singlesideband (SSB), and OCS [22–25]. Among them, there is still lack of comparison and investigation at a system level, especially for multiple-channel WDM-ROF systems up to 2.5 Gb per channel. Recently, an optical up-conversion for WDM-ROF transmission has been proposed [26]. The greatest limitation of this approach is that the generation of multiple carriers is too complicated. There are too many devices included in the system which may result in unnecessary loss of the whole link. Also, in practice, it is hard to control many parameters simultaneously to get optimal results. Besides, when MMW signals are transmitted through the fiber, chromatic dispersion should be considered for successful data transmission of all channels. SSB modulation is superior to DSB due to its reduced effects in suffering dispersion in optical fiber, but the receiver sensitivity is quite low. Although the OCS scheme cannot overcome the chromatic dispersion completely, either, it has been the one of attractive methods because of the higher receiver sensitivity, smaller power penalty and lower frequency requirement of radio frequency (RF) components compared with SSB modulation. Furthermore, the OCS modulation can be easily implemented with a large number of WDM channels simultaneously [27,28]. In this paper, we propose and demonstrate a new WDM-ROF scheme based on a FP laser with different modulation schemes to solve the issues mentioned above. Firstly, just a single FP laser is used as a WDM-ROF optical source is one of the highlights. Both the structure and cost of the system are greatly simplified compared with conventional schemes [12–14]. Furthermore, three novel architectures for WDM-ROF transmission have been compared to further improve system capacity and sensitivity. All modes from the FP laser are modulated simultaneously by a LNMZM for DSB, SSB, and OCS modulation, respectively. In this scheme, only one LN-MZM is used to generate MMW signals of seven channels. All devices except an FP laser are effective for spares because the modulators are colorless and only the exact mode spacing of FP laser has to be considered. Other previous methods are either more complicated or not easily applicable in WDM-ROF network [29–33]. The MMW generated by the OCS modulation scheme shows the highest receiver sensitivity, lowest spectral occupancy and smallest power penalty over a long delivery distance. In the simulation result, 7 WDM channels each carrying 7
2.5 Gb/s non-return-to-zero (NRZ) data have been simultaneously up-converted to 60 GHz with less than 2.5 dB power penalty. The average power penalty for all 7 WDM channels at a bit error rate (BER) of 10  9 after 25 km SMF transmission is about 2.7 dB. Good performances of BER and eye pattern also can

95

be achieved with a large operating range. Since our proposed systems do not use expansive sources and sophisticated technique, it reveals simpler and more economic advantages in the future WDM-ROF transmission networks.

2. Principle and discussions Fig. 1 shows the architecture of a typical 1:7 WDM-ROF cellular network. Microwave signals are delivered over an optical network from a CS to a number of BSs. All the optical signals are transmitted to BSs after up-conversion using external modulators. Generally, CS is composed of many laser sources operating in different wavelength. Thus the number of WDM channels is limited due to the difficulty of managing a large number of independent lasers which is too complicated and inevitably reduces the stability and re-configurability of the system. In order to ensure the reliability of the system at lower cost, a CS should be a simple, cost-effective, and colorless one without using expensive devices. Fig. 2 shows the proposed WDM-ROF system for simultaneous MMW generation and up-conversion. The system is mainly composed of FP laser, LN-MZM, RSOA, AWG and PD. Compared with conventional scheme (Fig. 1), the system is characterized by multiple light sources based on a FP laser. FP laser has many advantages such as narrow spectrum, high modulation rate, high stability and tunable frequency. Moreover, series of coherent multi-wavelength will appear and when laser operating current exceeds the current threshold, thus it makes all-optical up-conversion process for all WDM channels simple and inexpensive compared with using a DFB array or any other complicated cascaded devices. A frequency-tunable RF driving signal is applied to drive the LN-MZM to realize different modulation scheme. After amplification by an amplifier and transmission over SMF, the generated multiple OCS signals are divided into 7 individual channels by an AWG. In the BS, all the received signals are detected by a photodiode (PD) to realize up-conversion simultaneously. The optical amplitude and the power reflection coefficient of FP laser and air interface can be expressed as

n−1 n+1 ⎛ n − 1 ⎞2 ⎟ R1 = R2 = ⎜ ⎝ n + 1⎠

r1 = r2 =

(1)

In order to have enough gain to output laser, the light field in the cavity must satisfy the steady-state lasing condition:

⎡ r1r2 exp ⎢ − 2jβL − αL + Γ ⎣

∫0

Fig. 1. The architecture of a typical 1:7 WDM-ROF cellular network.

L

⎤ G (z ) dz⎥ = 1 ⎦

(2)

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Fig. 2. Schematic setup of MMW generation and up-conversion for WDM-ROF transmission (FP, Fabry-Perot; RF, radio frequency; LN-MZM, LiNbO3 Mach-Zehnder modulator; EA, electrical amplifier; RSOA, reflective semiconductor optical amplifier; SMF, single mode fiber; AWG, arrayed-waveguide grating; PD, photodiode; LO, local oscillator; EDA, eye diagram analyzer).

where β represents transmission constant, α represents scattering loss, Γ is the optical confinement factor. L and G(z) are defined as the length and the gain of FP cavity, respectively. After multi-longitudinal mode carriers are generated, the modulation dynamics of the laser are modeled by photon rate equation and carrier rate equation which describe a good mathematical model for a FP laser [34]:

dSi (t ) S (t ) = Γvg gi Si (t ) − + rs, i dt τp

dN (t ) I (t ) = − R (N ) − dt qV

n

∑ i =−n

Γvg gi Si (t ) V

(3)

(4)

where Si(t) is the photons density of mode i. When a photon is incident into the mode i, the radiation rate is vggi. rs,i is photon number of spontaneous emission. N(t) is the density of electrons in the conduction band. I(t) is the injection current, τp is the lifetime of photon. V is the active layer volume. q is the electron charge. R (N) is the rate of spontaneous recombination of carriers. Fig. 3 shows the spectra of the obtained comb generated by FP laser with different side modes. In this paper, we consider the length of the FP cavity as 0.06 cm and the reflectivity of the laser as 0.3. The threshold current of FP laser is about 18 mA. The central wavelength and the frequency interval of FP laser are 1545 nm and 30 GHz, respectively. It can be seen that the comb line count grows when the number of side modes is increased. Therefore, the amount and the frequency spacing of optical comb generated by the FP laser can be accurately controlled by dynamically adjust the

Fig. 3. Spectra of the obtained comb with different side modes n: (a) n ¼3; (b) n ¼5; (c) n ¼ 7; (d) n ¼ 10.

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Fig. 4. The output spectra of FP laser.

Fig. 5. The output spectra after multiple OCS modulation.

parameters of FP laser. The optical output power before and after the FP laser could be expressed respectively as [34]:

by the two arms of LN-MZM, respectively. Jn (∙) is the first kind Bessel function of nth order, β = VRF /Vπ is the modulation index of LN-MZM. Vπ is the half-wave voltage of LN-MZM. Then, the optical signals are amplified by an Erbium-doped Fiber Amplifier (EDFA) before transmission to RN through 25 km SMF. The fiber dispersion index is set as 16.75 ps/nm km, and the attenuation factor is set as 0.2 dB/km. The gain of EDFA is 20 dB and the noise figure is 4 dB. An RSOA is used both for the data modulation and for the reduction of MPN induced from FP laser. Two side carriers of each channel are modulated by an RSOA driven by 2.5-Gb/s pseudorandom binary sequence (PRBS) data with a word length of 231-1. In the RN, the generated multiple OCS signals are divided into 7 individual channels by an AWG with 30 GHz bandwidth and 5 dB loss in each output port. The mode spacing of the FP laser needs to be matched to that of AWG. At the receiver, a PD with responsivity R ¼1 A/W and 3 dB bandwidth of 60 GHz is used to up-convert CH1 to generate 60 GHz MMW signal. Fig. 6(a) and (b) show the detailed optical spectra of CH1 and the corresponding MMW after PD. The spacing between two adjacent first-order sidebands of two adjacent channels is large enough to mitigate the channel crosstalk effectively. Therefore, in the proposed 1:7 WDM-ROF system, all the 7 received signals are upconverted to 60 GHz MMW simultaneously. After being amplified by an EA, the signal is down-converted to baseband for EDA test by mixing it with a local oscillator (LO) signal. In order to evaluate the performance of the proposed WDMROF system, we investigate the BER performance of CH1 for backto-back (B-T-B, without SMF in the transmission link) and 25 km SMF transmission. The BER curves with inserted eye diagrams are shown in Fig. 7. It is indicated that the received optical power at the BER of 10  9 for B-T-B and 25 km SMF transmission are  34.1 dBm and  31.6 dBm, respectively. After 25 km, the eye still keep open a lot and the power penalty is 2.5 dB. Fig. 8 shows the receiver sensitivity of all 7 channels at a BER of 10  9 for B-T-B and 25 km SMF transmission. The average power penalty for all channels is about 2.7 dB after 25 km SMF transmission. For comparison, we also implement DSB and SSB modulation schemes to achieve optical up-conversion by using the same configuration as Fig. 2. For DSB modulation scheme, the LN-MZM is biased at 0. 5Vπ and there is a phase shift of 180°. The frequency

P1 =

P2 =

(1 − R1) R2 ( R1 +

R2 )(1 −

(1 − R2 ) R1 ( R1 +

R2 )(1 −

ηhω (I − Ith ) R1R2 ) q

(5)

ηhω (I − Ith ) R1R2 ) q

(6)

where R1 and R2 are power reflection coefficient before and after the FP cavity, ω is optical frequency, η is external differential quantum efficiency Ith is the threshold current. The sum of P1 and P2 is the total optical output power of FP laser. Fig. 4 shows the output spectra of FP laser with 1.37 nm FPmode spacing and the optical field at the output of FP laser can be expressed as

EFP ( t ) =

∑

Pi exp ( jωi t + ϕi ), Pi =

i

η0 hvi αm VSi Γ

(7)

where η0 is the differential quantum efficiency, vi is the optical frequency, αm is the mirror loss, V is the active layer volume, Si is the density of photons in the lasing mode. Then, all modes ( λ1 ∼ λ7) from FP laser are modulated by a RF subcarrier at a LN-MZM for different modulation schemes. The amplitude and the frequency of the RF source are 2.5 V and 30 GHz, respectively. Firstly, the LN-MZM is biased at Vμ to realize OCS modulation. The phase difference between two arms of LNMZM is 180°. The half-wave voltage is 4 V. The dc bias voltage difference is 4 V. The insertion loss and the extinction ratio of LNMZM are 5 dB and 30 dB, respectively. Fig. 5 shows the obtained optical spectra after multiple OCS modulation. As can be seen, a coherent 7 WDM channels (CH1–CH7) has been achieved. For one channel, assuming the optical field of the input signal into the LN-MZM is expressed as Ein(t) =E0 exp (j Ωt ), the optical field at the output of the LN-MZM can be given as

EOCS ( t ) = − 2e j

ϕ1+ ϕ 2 2 E0 J1 (βπ ) ⎡ ⎣ e j (Ω+ ωRF ) t

≈ 2E0 J1 (βπ ) ⎡⎣ e j (Ω+ ωRF ) t + e j (Ω− ωRF ) t ⎤⎦ where

+ e j (Ω− ωRF ) t ⎤⎦ (8)

Φ1 and Φ2 are defined as the fixed phase shift introduced

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C. Zhang et al. / Optics & Laser Technology 84 (2016) 94–101

Fig. 6. Optical spectra of (a) CH1 (2) the corresponding MMW after PD.

Fig. 7. BER curves of CH1 for B-T-B and 25 km SMF transmission (OCS scheme).

of the driven RF signal is 30 GHz. Fig. 9 shows the obtained optical spectra after multiple DSB modulation. Also assuming the optical field of the input signal into the LNMZM is expressed as Ein(t) =E0 exp (j Ωt ) for one channel, the optical field at the output of the LN-MZM can be given as

⎡π ⎤ ϕ1+ ϕ2 EDSB ( t ) = Ein cos ⎢ + βπ cos (ωRF t ) ⎥ e j 2 ⎣4 ⎦ ≈ E0 ⎡⎣ J (βπ ) e jΩt − J (βπ ) e j (Ω+ ωRF ) t − J (βπ ) e j (Ω− ωRF ) t ⎤⎦ 0

1

1

(9)

where Φ1 and Φ2 are defined as the fixed phase shift introduced by the two arms of LN-MZM, respectively. Jn ( ⋅) is the first kind Bessel function of nth order, β = VRF /Vπ is the modulation index of LN-MZM. Vπ is the half-wave voltage of LN-MZM. For DSB modulation, the transfer function of SMF which neglecting the nonlinear effect and loss can be expressed as

Fig. 8. Receiver sensitivity of all 7 channels for B-T-B and 25 km SMF transmission.

⎛ j ⎞ H ( ω) = exp ⎜ β2 ω2L⎟ ⎝2 ⎠

(10)

where β2 and L are the second-order dispersion parameter and the length of SMF, respectively. Assuming that the output of RF signal with baseband data is s ( t )=dc + m (t ) cos (ωRF t ), dc is direct current signal, m(t) is baseband signal. Thus the signal under the influence of the fiber dispersion can be expressed as

⎡ ⎛ j ⎞⎤ 2 sout ( t ) = ⎢ dc + m (t ) exp ⎜ β2 ωRF L⎟ ⎥ exp (j Ωt ) ⎝2 ⎠⎦ ⎣

(11)

When the signal is converted by PD, the current intensity of the output signal is proportional to the power of the optical signal:

IRF ∝ Pout ( t ) = sout ( t )

2

⎛1 ⎞ 2 ⎟ = dc + 2m (t ) cos ( ωRF t ) cos ⎜ β2 ωRF L ⎝2 ⎠

For the power of RF signal, there is a relational formula:

(12)

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Fig. 11. The output spectra after multiple SSB modulation.

Fig. 9. The output spectra after multiple DSB modulation.

the driven RF signal is also 30 GHz. Fig. 11 shows the obtained optical spectra after multiple SSB modulation. Also assuming the optical field of the input signal into the LNMZM is expressed as Ein(t) =E0 exp (j Ωt ) for one channel, the optical field at the output of the LN-MZM can be given as

ESSB ( t ) = ≈

1 jϕ2 ⎡ jβπ e Ein ⎣ je 2

π e jϕ2 E0 ⎡⎣ J0 (βπ ) e jΩt + j 4

cos ωRF t

+ e jβπ

sin (ωRF t + π ) ⎤

⎦

− J1 (βπ ) e j (Ω+ ωRF ) t ⎤⎦

(14)

where Φ1 and Φ2 are defined as the fixed phase shift introduced by the two arms of LN-MZM, respectively. Jn (∙) is the first kind Bessel function of nth order, β = VRF /Vπ is the modulation index of LN-MZM. Vπ is the half-wave voltage of LN-MZM. For SSB modulation, the signal under the influence of the fiber dispersion can be expressed as

⎛ j ⎞ 2 sout ( t ) = dc exp (j Ωt ) + m (t ) exp ⎡⎣ j (Ω + ωRF ) t ⎤⎦ exp ⎜ β2 ωRF L⎟ ⎝2 ⎠

(15)

When the signal is converted by PD, the current intensity of the output signal is proportional to the power of the optical signal: Fig. 10. BER curves of CH1 for B-T-B and 2 km SMF transmission (DSB scheme).

IRF ∝ Pout ( t ) = sout ( t ) ⎛1 ⎞ 2 2 ⎟ PRF ∝ IRF ≈ cos2 ⎜ β2 ωRF L ⎝2 ⎠

(13)

As can be seen, when (β2ω2RFL) ¼ π, PRF ¼0, the signal is completely distorted. What's more, the attenuation is more obvious with the increase of RF signal frequency. Therefore, DSB modulation is not suitable for long distance optical fiber transmission due to the severe periodic fading caused by the dispersion. The BER curves with eye diagrams of the DSB modulation scheme are shown in Fig. 10 for B-T-B and 2 km SMF transmission, respectively. As can be seen, the RF power is almost faded which may leads to a large power penalty after only 2 km SMF transmission. The power penalty is about 18 dB at a BER of 10  9. These results indicate that the DSB modulation-based scheme is not suitable for a large area access network. On the other hand, for SSB modulation scheme, the LN-MZM is biased at 0.25 Vπ and there is a phase shift of 90°. The frequency of

2

⎛ ⎞ 1 2 ⎟ = dc + 2m (t ) cos ⎜ ωRF t − β2 ωRF L ⎝ ⎠ 2

(16)

As can be seen, because there is only one sideband, the dispersion only affects the phase of the output signal and there is no effect on the signal amplitude which means that SSB modulation is less affected by the dispersion compared with DSB modulation. However, high dc component which comes from the central carrier will cause large carrier suppression ratio that may greatly reduce the sensitivity of receiver. While for OCS modulation, the output current intensity of the modulated signal after PD beat frequency can be expressed as:

IRF ∝ Pout ( t ) = sout ( t )

2

= dc +

1 2 m (t ) cos ( 2ωRF t ) 2

(17)

As can be seen, the dispersion has no effect on the phase and amplitude of the OCS modulated signal. Therefore, compared with DSB and SSB, the OCS modulation signal has higher receiving sensitivity and spectral efficiency which is suitable for long distance optical fiber transmission system.

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which is a feasible way for the WDM-ROF wireless access communication.

Acknowledgement This work is jointly supported by the National Natural Science Foundation of China (61405007, 61471033, 61525501), and the National Natural Science Foundation of Beijing (4154081).

References

Fig. 12. BER curves of CH1 for B-T-B and 20 km SMF transmission (OCS scheme).

The BER curves with eye diagrams of the SSB modulation scheme are shown in Fig. 12 for B-T-B and 25 km transmission, respectively. The power penalty is about 7.8 dB at a BER of 10  9 after 25 km transmission. Although SSB modulation is superior to DSB due to its reduced effects in suffering dispersion in optical fiber, the receiver sensitivity of SSB is quite low due to the large carrier to sideband ratio which contains many dc components at the center wavelength. Thus, the OCS scheme has the lowest received optical power among three schemes, which means this scheme shows the highest receiver sensitivity the smallest power penalty over long distance delivery.

3. Conclusion In conclusion, we have theoretically analyzed and numerically simulate a novel and cost-effective simultaneous MMW generation and up-conversion for WDM-ROF transmission using different multiple modulation schemes in FP laser. Unlike the previous reported WDM optical source generation, the proposed method is extremely simple because it requires only a single FP laser with its multi-carrier spectral characteristics. Tunability and the feasibility of the proposed WDM-ROF system have been discussed in the simulated verification and successfully proved by numerical simulation. In addition, we compare the performances of up-conversion based on different modulation schemes after all modes are modulated by a LN-MZM. The up-conversion signals based on OCS modulation scheme shows the best performance with the highest receiver sensitivity and the smallest power penalty over long distance delivery. In this scheme, 7 WDM channels each carrying 7
2.5 Gb/s baseband signal have been simultaneously up-converted to 60 GHz MMW over 25 km SMF with less than 2.5 dB power penalty. The average power penalty for all 7 WDM channels at a BER of 10  9 after 25 km SMF transmission is about 2.7 dB. The simulation results show that the eye diagrams still open a lot after long distance delivery. The key contributions are the simple operation for multiple optical up-conversion and high spectral efficiency, which significantly simplify the network structure and overall cost compared with the previous works. Therefore, the scheme has very cost-efficient configuration and good performance

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