A novel kind of programmable 3<sup>n</sup> feed-forward optical fiber true delay line based on SOA

doi:10.1364/OE.15.016760

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A novel kind of programmable 3ⁿ feed-forward optical fiber true delay line based on SOA

Xinwan Li, Limei Peng, Songbo Wang, Young-Chon Kim, and Jianping Chen »View Author Affiliations

Optics Express, Vol. 15, Issue 25, pp. 16760-16766 (2007)
http://dx.doi.org/10.1364/OE.15.016760

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▸ Article Outline
– Abstract
1. Introduction
2. Design of k feed-forward optical fiber true delay line
3. Implementation and performance measurement
4. Conclusions and discussion
– References and links

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Abstract

A new kind of 3ⁿ feed-forward programmable optical fiber true delay line was proposed. Theoretical analysis was presented on its delay performance and expandability. Experimental demonstration was given to show the implementation of such delay lines using SOAs and Farady rotation mirrors. Delay step as small as 0.5 ps with precision of about 0.03 ps was achieved. Measurement was performed to verify the feasibility and results are given.

1. Introduction

Programmable optical fiber delay line (OFDL) has attractive applications, such as antenna beam forming in phase array radar system [1

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

] and microwave signal processing[2

J. Capmany, B. Ortega, D. Pastor, and S. Sales, “Discrete-time optical Processing of microwave signals,” J. Lightwave Technol. 23, 702–723(2005) [CrossRef]

K. Wilner and A. P. Van Den Heuvel, “Fiber-optic delay lines for microwave signal processing,” Proc. IEEE 64, 805–807(1976) [CrossRef]

], due to various advantages in comparison with the traditional methods. It can provide true delay of microwave signal, very high time-bandwidth product, immunization to electromagnetic interference (EMI) and the possibility of spatial and wavelength parallelism using WDM techniques. So far, two types of optical fiber delay lines have been proposed as buffers for optical communication networks: re-circulating structure and traveling-wave structures [4

G. K. Chang, J. J Yu, Y. K. Yeo, A. Chowdhury, and Z. S. Jia, “Enabling technologies for next-generation optical packet-switching networks,” Proc. IEEE 94, 892–910(2006) [CrossRef]

]. Re-circulating buffers are compact and require fewer components to implement. However, the fiber loop length is fixed and hence it is not suitable for applications in phase array radar system and microwave signal processing. Traveling-wave buffer has the potential for the above applications. It can be cascaded to provide desired delay using, e.g., 2×2 optical switches or broadcast-and-select configuration [4

G. K. Chang, J. J Yu, Y. K. Yeo, A. Chowdhury, and Z. S. Jia, “Enabling technologies for next-generation optical packet-switching networks,” Proc. IEEE 94, 892–910(2006) [CrossRef]

]. Of course, high-speed (≤1ns) and low-loss (≤1dB) optical switches are required [1

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

A. J. Seeds, “Microwave photonics,” IEEE Transactions on Microwave Theory and Techniques 50, 877–887(2002) [CrossRef]

Two typical high speed devices, i.e. LiNbO₃ switch [6

E. J. Murphy, T. O. Murphy, A. F. Ambrose, R. W. Irvin, B. H. Lee, P. Peng, G. W. Richards, and A. Yorinks, “16×16 strictly nonblocking guided-wave optical switching system,” J. Lightwave Technol. 14, 352–358(1996) [CrossRef]

] and semiconductor optical amplifier (SOA) ON–OFF gate [4

G. K. Chang, J. J Yu, Y. K. Yeo, A. Chowdhury, and Z. S. Jia, “Enabling technologies for next-generation optical packet-switching networks,” Proc. IEEE 94, 892–910(2006) [CrossRef]

], can be considered. For LiNbO₃ 2×2 switch, the large insertion loss, polarization dependence and crosstalk need to be addressed before they can be used in phase array radar system and microwave signal processing. SOA ON–OFF gate has nanosecond switching speed, large on-off extinction ratio and wide optical bandwidth. It seems more suitable for traveling-wave type of fiber delay lines[7

Y.K. Yeo, J. J. Yu, and G. K. Chang, “A dynamically reconfigurable folded-path time delay buffer for optical packet switching,” IEEE Photonics Technol. Lett. 16, 2559–2561(2004) [CrossRef]

In this paper, we proposed a novel 3n feed-forward OFDL using SOAs as switching components. Theoretical analysis and experimental verification are presented. Important issues, such as the attenuation [8

B. Moslehi, “Fibre-optic filters employing optical amplifiers to provide design flexibility,” Electron. Lett. 28, 226–228(1992) [CrossRef]

], polarization [9

J. Capmany, B. Ortega, and D. Pastor, “A tutorial on microwave photonic filters,” J. Lightwave Technol. 24, 201–229(2006) [CrossRef]

,10

A. Ho-Quoc, S. Tedjini, and A. Hilt, “Optical polarization effect in discrete time fiber-optic structures for microwave signal processing,” IEEE MTT-S International Microwave Symposium Digest(1996),. pp.907–910

], delay precision, and etc., are addressed in detail.

2. Design of kⁿ feed-forward optical fiber true delay line

Traditionally, the programmable traveling-wave fiber delay line is implemented by using 1×N optical switch, or equally by using 1×N coupler plus on-off gate (e.g., SOA) in each arm, as shown in Fig. 1(a) [11

D.J. Blumenthal, P. R. Prucnal, and J. R. Sauer, “Photonic packet switches: architectures and experimental implementations,” Proc. IEEE 82, 1650–1667(1994) [CrossRef]

]. In this configuration, the maximum number of possible delays, y, is equal to the mount of SOAs, x. It is apparently not suitable to be scaled for large delays. The delay performance can be enhanced by configuring it in an alternative way with 2×2 switches(or 2×2 couplers plus ON-OFF SOA gates) and same number of SOAs as shown in Fig. 1(b). In this configuration, the total number of delays can be expressed as follows:

y = 2^{\frac{x}{2}}

(1)

where the number of SOAs, x, is supposed to even. We define such structure as 2ⁿ structure, where, n=x/2, is the number of stages.

Similarly, we can use x SOAs to build k^J structure as shown in Fig. 1(c), where k is the dimension of each stage and J is the number of stages(J=x/k is an integer). It can be derived the total number of delays, y, is given by:

y = k^{\frac{x}{k}}

(2)

Equation (2) can be rewritten as:

\frac{\log (y)}{x} = \frac{\log (k)}{k}

(3)

Equation (3) shows that for the same x, there is an optimal k that gives maximum y. We use L(k) to express the right side of Equation(3):

L (k) = \frac{\log (k)}{k}

(4)

The peak value of L(k) can be obtained mathematically by differential operation on L(k) as follows:

\frac{dL (k)}{dk} = 0

(5)

So, the k can be obtained by solving Equations (4) and (5):

k = e = 2.718

(6)

Fig. 1. The architectures of traveling-wave fiber delay lines (y=x for (a), y=2^x/2 for (b), and y=k^x/k for (c), where y is the maximum number of possible delays, and x is the number of SOAs)

However, as shown in Fig. 1, the physics meaning of k is the number of delay lines in each stage, so, k should be integer. Figure 2 shows the relationship between L(k) and k. It can be easily found that the optimal value k is 3. It can also be proved that 4 ⁿ structure is exactly the same as 2 ⁿ structure:

y (x) = k^{\frac{x}{k}} \overset{k = 4}{\to} y (x) = 4^{\frac{x}{4}} = {(2^{2})}^{\frac{x}{4}} = 2^{2 • \frac{x}{4}} = 2^{\frac{x}{2}} \to k = 2

(7)

When the number of SOAs used is small, the difference between the maximum numbers of delays available for k=2 and k=3, respectively, are also small. However, as more SOAs are used, the difference becomes significant, as shown in Fig. 3. From the figure, we can see that fast tunable true delay line with large number of delays can be obtained by employing 3 ⁿ structure and certain number of SOAs.

Fig. 2. L(k) as a function of k

Fig. 3. Dependence of total delay number on the delay units per stage (x: the number of SOAs)

3. Implementation and performance measurement

According to the above analysis, 3 ⁿ feed-forward optical fiber true delay line based on SOAs can provides large amount of delays with fast tunability and easier scalability. For practical applications, problems such as insertion loss and polarization dependence should be solved. The former can be overcome by SOA itself for it provides gain in addition to ON-OFF gating. In fact, this is also one of advantages of employing SOAs. We use Farady rotation mirrors (FRMs) to alleviate the polarization dependence in optical fiber [12

C.H. Shi, J.P. Chen, G.L. Wu, and X.W. Li, “Stable dynamic detection scheme for magnetostrictive fiber-optic interferometric sensors,” Opt. Express 14: 5098–5102(2006), http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-12-5098 [CrossRef] [PubMed]

] and in SOAs. The SOAs we used are bi-directional SOA without inside optical isolators at input and output. Due to the low backward reflection of the SOA and the polarization rotation by FRM, no oscillation was observed. Figure 4 shows the structure for practical implementation and we built a two-stage (the first two stages) feed-forward optical fiber true delay line for experimental verification. For each stage, the light is fed from the input port of the optical circulator. It experiences delay whose amount is determined by turning on one of the SOAs and is then forwarded to the next stage via the output port.

The main insertion loss is caused by 1×3 coupler. The light beam passes the coupler twice and experience about 12 dB loss (excluding the coupler’s excess loss). By taking the losses resulted from the FRM and the circulator into consideration, the total loss of each stage is around 13 dB. It should be noted that the beam also passes SOA twice. Usually, an SOA can provide with 5–8 dB gain for small optical signals, e.g. -10dBm, under moderate drive current (~80mA). Therefore, by careful design of the drive current of each SOA, it is possible to obtain a delay stage with no insertion loss. This feature is particularly important for practical applications. It enables the proposed structure to be cascaded without limitation on the stage number.

Fig. 4. A practical structure of 3 ⁿ feed-forward optical fiber true delay line

The resolution is another key issue to be addressed. The delay introduced by a light path distance of 0.15mm, or fiber length of 0.103mm (group refractive index of 1.46 is supposed), is about 0.5 ps. It corresponds to a phase control precision of about 1.8° in X-band (λ~3cm). In order to reach the above precision level, we used hydrogen flame to precisely adjust the fiber length, with the help of the high precision reflectometer (HP8504)[12

]. This equipment is used to precisely measure the lightpath difference between the measured arm and reference arm of Michelson interferometer. The measurement span (equivalent distance in air) is 1–400 mm with resolution of 20 µm. Figure 5 shows the result when we set all six SOAs ON. It can be clearly seen that there are 9 peaks, which represent the all possible delays to be obtained, and each delay can be programmablely switched by setting of the relative one SOA per stage. The measured relative light path distances in three arms of the first stage are 20.756mm, 20.914mm, 21.056mm, respectively. The correspondent fiber lengths are 14.216 mm, 14.325mm, 14.422mm, respectively. These data indicate that τ as shown in Fig. 3 is 0.5 ps and the precision is kept within 0.03ps. One may note that there are insertion losses in stages 2 and 3. This is because of the limited drive current of the circuit we used, resulting in small SOA gain incapable to compensate the inherent insertion loss mentioned above.

Figure 6 shows the measured result of tuning speed. The rise time is 18.6 ns and the fall time id 39.4 ns. Such speed can enhance the scanning performance of microwave significantly. The scanning frequency may be improved from tens of kHz up to 10 MHz. The drive circuit for the SOAs was implemented with conventional electronic devices at hand. There is large potential to improve the tuning speed, especially the switching-off performance.

One may note that there is extra delay in switching-off process, which is mainly resulted in from the large junction capacitance in the device.

We also measured the polarization dependence loss (PDL) using polarization controller. The measured result is 0.18 dB. In comparison with the PDL of SOA (~0.3dB), there is improvement that is mainly due to the employment of FRM.

The relatively low noise figure of the SOA may bring a certain limitation to the proposed structure for some applications, especially for those requiring high transmission SNR. This indicates that for expansion purpose, noise performance should be taken into consideration, which is a topic worthy of intensive study and, however, beyond the scope of the paper.

Fig. 5. Measured result of relative fiber lengths x-axis: light path difference(0.2 mm/div), y-axis: relative intensity of the output(dB)

Fig. 6. Photograph of tuning speed measurement (lower trace: drive pulse, upper trace: optical output)

4. Conclusions and discussion

In this paper, we proposed a new kind of 3 ⁿ feed-forward optical fiber delay lines. Theoretical analysis shows that it can provide large delays and is easy to be expanded. Experimental demonstration to implement such delay lines using SOAs and Farady rotation mirrors is presented. Delay step as small as 0.5 ps with precision of about 0.03 ps was realized. Measured results verify the feasibility of the proposed method. Its excellent expandability, low insertion loss, low polarization dependence loss and high tuning speed are suitable for application in phase array radar system and microwave signal processing.

Acknowledgments

The work was partially supported by 863 Project (ID: 2006AA01z242, 2007AA01Z275), Dawn Program for Excellent Scholars by Shanghai Municipal Education Commission, New Century Program for Excellent Scholars by Education of Ministry of China, National Spaceflight Innovation Fund of China (2007 Key Project), Shanghai Municipal Science and Technology Commission Project (05DZ22312), and partially supported by the MIC(Ministry of information and Communication), Korea, Under the ITFSIP(IT Foreign Specialist Inviting Program) supervised by the IITA (Institute of Information Technology Advancement).

References and links

1.	A. J. Seeds and K. J. Williams, “Microwave Photonics,” J. Lightwave Technol. 24, 4628–4641(2006) [CrossRef]
2.	J. Capmany, B. Ortega, D. Pastor, and S. Sales, “Discrete-time optical Processing of microwave signals,” J. Lightwave Technol. 23, 702–723(2005) [CrossRef]
3.	K. Wilner and A. P. Van Den Heuvel, “Fiber-optic delay lines for microwave signal processing,” Proc. IEEE 64, 805–807(1976) [CrossRef]
4.	G. K. Chang, J. J Yu, Y. K. Yeo, A. Chowdhury, and Z. S. Jia, “Enabling technologies for next-generation optical packet-switching networks,” Proc. IEEE 94, 892–910(2006) [CrossRef]
5.	A. J. Seeds, “Microwave photonics,” IEEE Transactions on Microwave Theory and Techniques 50, 877–887(2002) [CrossRef]
6.	E. J. Murphy, T. O. Murphy, A. F. Ambrose, R. W. Irvin, B. H. Lee, P. Peng, G. W. Richards, and A. Yorinks, “16×16 strictly nonblocking guided-wave optical switching system,” J. Lightwave Technol. 14, 352–358(1996) [CrossRef]
7.	Y.K. Yeo, J. J. Yu, and G. K. Chang, “A dynamically reconfigurable folded-path time delay buffer for optical packet switching,” IEEE Photonics Technol. Lett. 16, 2559–2561(2004) [CrossRef]
8.	B. Moslehi, “Fibre-optic filters employing optical amplifiers to provide design flexibility,” Electron. Lett. 28, 226–228(1992) [CrossRef]
9.	J. Capmany, B. Ortega, and D. Pastor, “A tutorial on microwave photonic filters,” J. Lightwave Technol. 24, 201–229(2006) [CrossRef]
10.	A. Ho-Quoc, S. Tedjini, and A. Hilt, “Optical polarization effect in discrete time fiber-optic structures for microwave signal processing,” IEEE MTT-S International Microwave Symposium Digest(1996),. pp.907–910
11.	D.J. Blumenthal, P. R. Prucnal, and J. R. Sauer, “Photonic packet switches: architectures and experimental implementations,” Proc. IEEE 82, 1650–1667(1994) [CrossRef]
12.	C.H. Shi, J.P. Chen, G.L. Wu, and X.W. Li, “Stable dynamic detection scheme for magnetostrictive fiber-optic interferometric sensors,” Opt. Express 14: 5098–5102(2006), http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-12-5098 [CrossRef] [PubMed]

OCIS Codes
(060.2340) Fiber optics and optical communications : Fiber optics components
(120.4610) Instrumentation, measurement, and metrology : Optical fabrication
(220.4830) Optical design and fabrication : Systems design
(230.7020) Optical devices : Traveling-wave devices
(350.4010) Other areas of optics : Microwaves

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: August 22, 2007
Revised Manuscript: October 3, 2007
Manuscript Accepted: October 22, 2007
Published: December 3, 2007

Citation
Xinwan Li, Limei Peng, Songbo Wang, Young-Chon Kim, and Jianping Chen, "A novel kind of programmable 3ⁿ feed-forward optical fiber true delay line based on SOA," Opt. Express 15, 16760-16766 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-25-16760

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References

A. J. Seeds, K. J. Williams, "Microwave Photonics," J. Lightwave Technol. 24, 4628-4641(2006) [CrossRef]
J. Capmany, B. Ortega, D. Pastor, S. Sales, "Discrete-time optical Processing of microwave signals," J. Lightwave Technol. 23, 702-723(2005) [CrossRef]
K. Wilner and A. P. Van Den Heuvel, "Fiber-optic delay lines for microwave signal processing," Proc. IEEE 64, 805-807(1976) [CrossRef]
G. K. Chang, J. J Yu, Y. K. Yeo, A. Chowdhury, Z. S. Jia, "Enabling technologies for next-generation optical packet-switching networks," Proc. IEEE 94, 892-910(2006) [CrossRef]
A. J. Seeds, "Microwave photonics," IEEE Transactions on Microwave Theory and Techniques 50, 877- 887(2002) [CrossRef]
E. J. Murphy, T. O. Murphy, A. F. Ambrose, R. W. Irvin, B. H. Lee, P. Peng, G. W. Richards, A. Yorinks, "16×16 strictly nonblocking guided-wave optical switching system, " J. Lightwave Technol. 14, 352- 358(1996) [CrossRef]
Y.K. Yeo, J. J. Yu, G. K. Chang, "A dynamically reconfigurable folded-path time delay buffer for optical packet switching," IEEE Photonics Technol. Lett. 16, 2559 - 2561(2004) [CrossRef]
B. Moslehi, "Fibre-optic filters employing optical amplifiers to provide design flexibility," Electron. Lett. 28, 226-228(1992) [CrossRef]
J. Capmany, B. Ortega, D. Pastor, "A tutorial on microwave photonic filters," J. Lightwave Technol. 24, 201-229(2006) [CrossRef]
A. Ho-Quoc, S. Tedjini, A. Hilt, "Optical polarization effect in discrete time fiber-optic structures for microwave signal processing," IEEE MTT-S International Microwave Symposium Digest(1996), pp.907-910
D.J. Blumenthal, P. R. Prucnal, J. R. Sauer, "Photonic packet switches: architectures and experimental implementations," Proc. IEEE 82, 1650-1667(1994) [CrossRef]
C.H. Shi, J.P. Chen, G.L. Wu, X.W. Li, "Stable dynamic detection scheme for magnetostrictive fiber-optic interferometric sensors," Opt. Express 14: 5098-5102(2006), http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-12-5098 [CrossRef] [PubMed]

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