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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 19 — Sep. 17, 2007
  • pp: 12167–12173
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Simultaneous noise and distortion reduction of a broadband optical feedforward transmitter for multi-service operation in radio-over-fiber systems

Yon-Tae Moon, Jun Woo Jang, Woon-Kyung Choi, and Young-Wan Choi  »View Author Affiliations


Optics Express, Vol. 15, Issue 19, pp. 12167-12173 (2007)
http://dx.doi.org/10.1364/OE.15.012167


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Abstract

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

1. A. J. Cooper, “Fibre/radio for the provision of cordless/mobile telephony services in the access network,” Electron. Lett. 26, 2054–2056 (1990). [CrossRef]

,2

2. A. J. Seeds and K. J. Williams, “Microwave photonics,” J. Lightw. Technol. 24, 4628–4641 (2006). [CrossRef]

].

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

7. L. S. Fock and R. S. Tucker, “Simultaneous reduction of intensity noise and distortion in semiconductor lasers by feedforward compensation,” Electron. Lett. 27, 1297–1299 (1991). [CrossRef]

,8

8. D. Hassin and R. Vahldieck, “Feedforward linearization of analog modulated laser diodes-theoretical analysis and experimental verification,” IEEE Trans. Microw. Theory Tech. 41, 2376–2382 (1993). [CrossRef]

]. 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.

Fig. 1. Broadband feedforward compensation scheme.

ΔIMD=10log1+10ΔA102×10ΔA20cos(Δϕ)
(1)

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

11. R. S. Tucker, “Linearization techniques for wideband analog transmitters,” Broadband Analog and Digital Optoelectronics, Optical Multiple Access Networks, Integrated Optoelectronics, Smart Pixels, LEOS 1992 Summer Topical Meeting Dig., 54–55 (1992).

].

Table 1. Measured results for carrier frequencies

table-icon
View This Table

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’.

References and links

1.

A. J. Cooper, “Fibre/radio for the provision of cordless/mobile telephony services in the access network,” Electron. Lett. 26, 2054–2056 (1990). [CrossRef]

2.

A. J. Seeds and K. J. Williams, “Microwave photonics,” J. Lightw. Technol. 24, 4628–4641 (2006). [CrossRef]

3.

P. K. Tang, L. C. Ong, B. Luo, A. Alphones, and M. Fujise, “Transmission of multiple wireless standards over a radio-over-fiber network,” 2004 IEEE MTT-S International, 3, 2051–2054 (2004).

4.

P. Hartmann, A. Bothwell, R. Cronin, K. Leeson, A. Loveridge, D. C. Parkinson, J. W. Ure, R. V. Penty, I. H. White, and A. J. Seeds, “Wideband fibre-agnostic DAS using pluggable analogue optical modules,” Microwave Photonics 2006, P5 (2006).

5.

Hung-Tser Lin and Yao-Huang Kao, “Nonlinear distortions and compensations of DFB laser diode in AMVSB lightwave CATV applications,” J. Lightw. Technol. 14, 2567–2574 (1996). [CrossRef]

6.

L. Roselli, V. Borgioni, F. Zepparelli, F. Ambrosi, M. Comez, P. Faccin, and A. Casini, “Analog laser predistortion for multiservice radio-over-fiber systems,” J. Lightw. Technol. 21, 1211–1223 (2003). [CrossRef]

7.

L. S. Fock and R. S. Tucker, “Simultaneous reduction of intensity noise and distortion in semiconductor lasers by feedforward compensation,” Electron. Lett. 27, 1297–1299 (1991). [CrossRef]

8.

D. Hassin and R. Vahldieck, “Feedforward linearization of analog modulated laser diodes-theoretical analysis and experimental verification,” IEEE Trans. Microw. Theory Tech. 41, 2376–2382 (1993). [CrossRef]

9.

Hyun-Do Jung and Sang-kook Han, “Nonlinear distortion suppression in directly modulated DFB-LD by dual-parallel modulation”, IEEE Photon. Technol. Lett. 14, 980–982 (2002). [CrossRef]

10.

A. Kaszubowska, P. Anandarajah, and L.P. Barry, “Improved performance of a hybrid radio/fibre system using a directly modulated laser transmitter with external injection,” IEEE Photon. Technol. Lett. 14, 223–235 (2002). [CrossRef]

11.

R. S. Tucker, “Linearization techniques for wideband analog transmitters,” Broadband Analog and Digital Optoelectronics, Optical Multiple Access Networks, Integrated Optoelectronics, Smart Pixels, LEOS 1992 Summer Topical Meeting Dig., 54–55 (1992).

OCIS Codes
(060.2330) Fiber optics and optical communications : Fiber optics communications
(130.0250) Integrated optics : Optoelectronics
(140.3490) Lasers and laser optics : Lasers, distributed-feedback

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: July 9, 2007
Revised Manuscript: September 3, 2007
Manuscript Accepted: September 3, 2007
Published: September 10, 2007

Citation
Yon-Tae Moon, Jun Woo Jang, Woon-Kyung Choi, and Young-Wan Choi, "Simultaneous noise and distortion reduction of a broadband optical feedforward transmitter for multi-service operation in radio-over-fiber systems," Opt. Express 15, 12167-12173 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-19-12167


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References

  1. A. J. Cooper, "Fibre/radio for the provision of cordless/mobile telephony services in the access network," Electron. Lett. 26, 2054-2056 (1990). [CrossRef]
  2. A. J. Seeds and K. J. Williams, "Microwave photonics," J. Lightw. Technol. 24, 4628-4641 (2006). [CrossRef]
  3. P. K. Tang, L. C. Ong, B. Luo, A. Alphones, and M. Fujise, "Transmission of multiple wireless standards over a radio-over-fiber network," 2004 IEEE MTT-S International, 3, 2051-2054 (2004).
  4. P. Hartmann, A. Bothwell, R. Cronin, K. Leeson, A. Loveridge, D. C. Parkinson, J. W. Ure, R. V. Penty, I. H. White, and A. J. Seeds, "Wideband fibre-agnostic DAS using pluggable analogue optical modules," Microwave Photonics 2006, P5 (2006).
  5. H.-T. Lin and Y.-H. Kao, "Nonlinear distortions and compensations of DFB laser diode in AM-VSB lightwave CATV applications," J. Lightw. Technol. 14, 2567-2574 (1996). [CrossRef]
  6. L. Roselli, V. Borgioni, F. Zepparelli, F. Ambrosi, M. Comez, P. Faccin, and A. Casini, "Analog laser predistortion for multiservice radio-over-fiber systems," J. Lightw. Technol. 21, 1211-1223 (2003). [CrossRef]
  7. L. S. Fock and R. S. Tucker, "Simultaneous reduction of intensity noise and distortion in semiconductor lasers by feedforward compensation," Electron. Lett. 27, 1297-1299 (1991). [CrossRef]
  8. D. Hassin and R. Vahldieck, "Feedforward linearization of analog modulated laser diodes-theoretical analysis and experimental verification," IEEE Trans. Microw. Theory Tech. 41, 2376-2382 (1993). [CrossRef]
  9. Hyun-Do Jung, and Sang-kookHan , "Nonlinear distortion suppression in directly modulated DFB-LD by dual-parallel modulation", IEEE Photon. Technol. Lett. 14, 980-982 (2002). [CrossRef]
  10. A. Kaszubowska, P. Anandarajah and L.P. Barry, "Improved performance of a hybrid radio/fibre system using a directly modulated laser transmitter with external injection," IEEE Photon. Technol. Lett. 14, 223-235 (2002). [CrossRef]
  11. R. S. Tucker, "Linearization techniques for wideband analog transmitters," Broadband Analog and Digital Optoelectronics, Optical Multiple Access Networks, Integrated Optoelectronics, Smart Pixels, LEOS 1992 Summer Topical Meeting Dig., 54-55 (1992).

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