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The broadband optical feedforward transmitter with uncooled and unisolated distributed-feedback laser diodes (DFB LDs) is developed for a radio-over-fiber system. Although we use DFB LDs for digital communications, the feedforward compensation method can significantly suppress the intermodulation distortions and background noise. For the wide frequency range from 2.05 to 2.60 GHz (550 MHz), the third-order intermodulation distortion (IMD3) is suppressed by more than 10 dB. We also analyze the variation of IMD3 and noise for the bias current of LDs. With the linearization technique, the maximum IMD3 suppression and spurious-free dynamic range enhancement are 21.3 dB and 7.11 dB, respectively, at 2.3 GHz.
©2007 Optical Society of America
1. Introduction
Radio-over-Fiber (RoF) technologies are attracting much attention from radio network operators and manufacturers for the deployment of flexible and cost-effective radio access system. For the emerging broadband wireless standards, in particular, operating at carrier frequencies below 3 GHz, one of the main challenges of RoF techniques is how to transport the multiple wireless standards over fiber-optic radio links. In fiber-optic radio system, the cost of the optical transmitter that modulates and multiplexes signals typically dominates the system cost [1,2].
Recently, transmission characteristics of multiple services have been reported for use in an electro-absorption modulator [3] and a pluggable analogue optical module with a highly linear distributed-feedback laser diode (DFB LD) [4]. Although high performances for wireless standards in multiple services have been realized by previous works, it is not a complete solution to use the external modulator and the high-linearity DFB laser fabricated by a complex process in flexible and cost-effective optical transmission systems.
To develop a suitable system carrying a number of different services simultaneously, we propose the use of uncooled DFB LD for digital applications. A commercial semiconductor laser for digital communications is characterized by poor linearity. This limitation, however, can be overcome by using compensation techniques. Generally, because the nonlinear behavior of commercial LDs causes distortions in the analog signals that are transmitted over an analog fiber-optic link, linearization techniques must be used for improving the linearity performance for direct modulation. Several methods to reduce the distortion signals of LD have been proposed [5–10]. Of the traditional optoelectronic linearization techniques, the low-cost broadband analog predistortion circuits are investigated to reduce harmonic distortions and intermodulation of lasers used in the multiservice system [5,6]. Remarkable results show that its practical performance is limited to 2 GHz due to parasitic parameters of predistortion circuit. However, the feedforward compensation method has advantages to reduce the distortions and relative intensity noise (RIN) of Fabry-Perot LD over a wide range of services [7,8]. In an analog subcarrier multiplexed system (SCM), the high linearity, reduced noise, and large dynamic range are needed for better link performance.
In this work, the intermodulation distortion products and noise floor of digital DFB laser on employing optical feedforward scheme are measured and observed by the variation of optical modulation index (OMI) in multiservices RoF systems. The third-order intermodulation distortion products (IMD3), RIN, and spurious-free dynamic range (SFDR) over various carrier frequencies are measured as the evaluation criteria for the performance. The implemented system accommodates four different RF services, which operate within the frequency range 2.0 GHz~2.7 GHz, thus providing an infrastructure for wide range of services, such as 2.5G, wireless broadband (WiBro), the ISM-band wireless LANs, and digital multimedia broadcasting (DMB).
2. Broadband feedforward linearization scheme
The broadband feedforward compensation scheme is illustrated in Fig. 1. The proposed scheme is composed of 180° hybrid coupler, radio-frequency (RF) attenuators, RF amplifiers, electrical delay lines, optical couplers with 60:40 coupling ratio, a photodiode, and two laser diodes which are digital DFB LDs without cooler and isolator. We used a 180° hybrid coupler to achieve operation over a larger bandwidth against the reported works [7,8]. The characteristic impedance between a broadband bias-T and coaxial lead lines of LD is effectively matched on the printed circuit board. The wavelengths of DFB LD1 (1549.20 nm) and DFB LD2 (1548.66 nm) are spaced about 0.54 nm. The phase mismatch is caused by fiber dispersion as a function of wavelength. By appropriately adjusting the optical and electrical delay line, the phase error by wavelength difference can be eliminated at the end. When the bias current of LD1 and LD2 is 14 mA and 13 mA, the optical power is -5.3 dBm and -5.8 dBm, respectively.
The optical coupler with angled physical contact (APC) polishing type is used for making the unisolated LD stable against the back scattered optical power. The feedforward compensation scheme consists of two cancellation loops. The main loop is the carrier signal cancellation loop which is used to reduce the carriers and extracts the IMD3 error signal of the primary LD through amplitude and phase adjustment. The second loop is the error cancellation loop. The error signal obtained in the previous loop is appropriately amplified, and drives the secondary LD to cancel the IMD3 generated by the primary LD. By adding the IMD3 error signal with 180 degree phase shift to the signal of the primary LD, the suppression of the distortion products is achieved [7,8]. The key aspects for broadband and significant suppression are the amplitude and phase match as well as the equality of the signal delays between the different branches. The signal delays are matched by controlling the electrical delay line. The amount of the correction is limited by the ability of the two loops to match amplitude and phase between the main signal and error paths. When only a small amount of amplitude and phase error is achieved, the correction is determined by
where ΔIMD is the amount of IMD improvement in dB, ΔA is the amplitude error between output of the primary LD and output of error cancellation, and Δϕ is the phase error between amplitude of error cancellation and output of the primary LD. Because the IMD cancellation in each loop is limited by the error value of amplitude and phase, the amplitude and phase balance of RF components and optical devices need to be strictly maintained to achieve broadband suppression over wide range.
3. Results and discussion
A broadly linearized system that covers RF band for multiservice operation would be desirable in RoF systems. Figure 2 shows the detected RF power with and without feedforward linearization method in various RF regions. The applied RF band was from 2.0 to 2.7 GHz with 1 MHz channel spacing. With the optimization of amplitude and phase of the two loop paths, the IMD3 can be simultaneously suppressed by more than 10 dB at a very wide frequency range of 550 MHz (2.05~2.60 GHz) compared without the linearization technique.
The OMI of the directly modulated LD1 is represented by m=(Δi ac/2)/(I dc-I th), where Δi ac is the input RF power, I dc is the biased current from 11.750 to 15.125 mA and I th is the threshold current, 9.8 mA. The OMI varied from 0.31 to 0.86 for a fixed input RF power and a threshold current. The IMD3 and noise as function of the OMI of the LD1 at 2.3 and 2.4 GHz are shown in Fig. 3(a).
Fig. 2. Third-order intermodulation distortion (IMD) products with and without feedforward linearization method.
Fig. 3. Variation of IMD3 and background noise as functions of (a) the optical modulation index (OMI) of the LD1 (b) the biasing current of LD2 at 2.3 and 2.4 GHz.
As the OMI is increased, the LD1 is biased relatively close to threshold region. The suppressed IMD3 is slightly reduced, but the significant reduction in the intensity noise is obtained due to the higher noise level generated by spontaneous emission. As the OMI is decreased, IMD3 is significantly suppressed and noise is slightly reduced. For the optimized performance, the operating current of LD1 and LD2 is correctly selected by IMD3 and noise factor at 2.3 and 2.4 GHz. When OMI is fixed as 0.4 and other conditions are fixed, the reduced IMD3 and noise as a function of the variable LD2 current are shown in Fig. 3(b).
The reduction of IMD3 and noise is highest at the biasing current of LD2 with 13 mA, which corresponds to optimized condition in the system. Figure 4(a) shows the measured power spectrum of the LD1 modulated by a two-tone microwave signal with -3 dBm.
Fig. 4. (a) Simultaneously suppressed intermodulation distortion and noise, and (b) spurious-free dynamic range (SFDR) with and without feedforward compensation at 2.3 GHz. The background noise floor with and without feedforward are -134.48 dBm/Hz, -132.91 dBm/Hz, respectively.
The two-tone frequency separation Δf is kept as 8 MHz. Without feedforward compensation the IMD3 is measured to be -19.6 dBc and with feedforward the IMD3 is considerably reduced to -40.9 dBc. The experimental result shows that the IMD3 and noise are significantly reduced about 21.3 dB, 1.57 dB, respectively. From Fig. 4(b), we can estimate SFDR of directly modulated DFB LD by linear-fitting. When the system noise floor with and without feedforward are -134.48 dBm/Hz, -132.91 dBm/Hz, respectively, the SFDR with compensation is about 86.84 dB·Hz2/3 and without compensation is about 79.73 dB·Hz2/3. With feedforward scheme, we can achieve 7.11 dB dynamic range enhancement at 2.3 GHz in implemented system. The measured results for carrier frequencies are summarized in Table 1. IMD3, noise, and SFDR with channel spacing for each multi service are measured.
In conventional LD modules, thermoelectric coolers and optical isolators are inevitable components for minimizing the temperature and reflection dependence. However, the optical feedforward transmitter has effectiveness for the distortion characteristics, although the uncooled and unisolated LDs in the system are used [11].
Table 1. Measured results for carrier frequencies
4. Conclusion
The broadband optical feedforward transmitter for use in multi-services RoF systems has been developed. The measured results show 21.3 dB IMD3 suppression, 1.57 dB RIN reduction, and 7.11 dB SFDR enhancement by 86.8 dB·Hz2/3 at 2.3 GHz. The simultaneously suppressed IMD3 in the feedforward linearization is by more than 10 dB at an ultra wide frequency range of 550 MHz (2.05~2.60 GHz) compared without linearization. As measured results for multi carrier frequencies, the implemented transmitter capable of reducing the IMD3 of 7–12 dB and noise of about 1–2 dB, and improving SFDR of about 2–7 dB has been obtained in feedforward system. These results indicate that our broadband optical feedforward transmitter is very suitable and flexible in advanced RoF systems for next-generation mobile communications.
Acknowledgement
This work was partially supported by ‘Seoul R&BD Program (10544)’ and ‘grant No.(R01-2005-000-10176-0) from the Basic Research Program of the Korea Science & Engineering Foundation’.
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