1. Introduction

An all-optical switching device is a key component to accomplish all-optical communications systems and so the investigations on the fast switching phenomenon were intensively carried out [1,2]. Recently, much attention has been paid to the fiber-type all-optical switching devices using the high nonlinear optical properties of a doped fiber, such as a fiber Mach-Zehnder(MZ) interferometer, a twin-core fiber(TCF) interferometer, a two-mode fiber(TMF) interferometer [1]. However, the fiber MZ interferometer is sensitive to the thermal effect due to non-radiative relaxation processes in the dopant of the fiber [3] and thus it causes an undesirable phase change in the switching operation. On the other hand, even though the TCF and TMF interferometers are insensitive to the thermal effect, the former is not easy to fabricate such a twin-core fiber and the latter has a limitation that signal wavelengths must be in the two-mode region. Moreover, the TCF and TMF interferometers need bulk optics at input or output ports of the fibers to operate in the system.

We propose, in this paper, a new fiber-type switching device employing a long-period fiber gratings(LPG) pair with the doped fiber having a very high optical nonlinearity. The LPG configuration we adopt is insensitive to the thermal effect of the doped fiber because of the one-fiber operation during all-optical switching in principle [4,5]. A LPG allows a part of the incident beam, which has been guided only along the fiber core, to couple into several co-directional cladding modes that satisfy the phase matching condition of the gratings, so the transmission spectrum of the LPG has a set of stop bands at different wavelengths [6]. If there is another LPG next to the first LPG along the fiber, the coupled cladding modes can be re-coupled into the core mode at the second LPG. Thus a transmission spectrum of the light beam passing through the pair of the LPG has an interference fringe pattern at the stop bands due to interference between the core-guided light and the re-coupled cladding-guided light [7]. When a doped fiber with large optical nonlinearity is inserted between the LPG pair, a shift of the interference fringe is expected because the refractive index of the nonlinear optical fiber core increases upon pumping. Therefore, the fringe shift can be controlled optically, which is essential for optical switching application, by changing the pump power. We demonstrated the all-optical ON-OFF switching at ~1556nm using a LPG pair with a Yb³⁺ doped aluminosilicate glass optical fiber upon pumping with the square wave beam pulses at 976nm.

2. Experiments

A preform of high concentration Yb³⁺ doped aluminosilicate glass fibers for all-optical switching was fabricated by using the modified solution doping technique [8], in which the usual drain process was omitted, using an aqueous solution of YbCl₃(0.05M) and AlCl₃(0.24M) as a doping solution in the modified chemical vapor deposition(MCVD) process. The preform was drawn into fibers. The core diameter of the Yb³⁺ doped fiber, its absorption coefficient at the pumping wavelength of 976nm, and its cut-off wavelength were 4.4µm, 2.8cm^-1, and 1.15µm, respectively. The concentration of ytterbium ions and aluminum ions in the fiber core was ~0.82at% and ~3.02at%, respectively, which were measured by the electron probe micro-analyzer(EPMA).

A LPG pair was written on the conventional single mode fiber(SMF) using the amplitude mask of 450µm period. Before writing the grating, the SMF was hydrogen loaded at 100°C under a pressure of 10 MPa for a week. After the grating formation on the bare fiber with the KrF excimer laser(248 nm), fibers were annealed at 100°C for 24 h. Then, the Yb³⁺ doped fiber was spliced between a pair of LPG as shown in Fig. 1(a). The total length, L, between the LPG pair including the Yb³⁺ doped fiber was 30.2cm, where the length, L1, of the Yb³⁺ doped fiber was 25.5cm. Because the LPG pair splicing with doped fibers is sensitive to the polarization of the incident light [9], the polarization state from the continuous wave(CW) tunable laser source(TLS; TSL-200, Santec) was controlled to maximize the extinction ratio of the interference fringe pattern formed by the LPG pair with the Yb³⁺ doped fiber by using the polarization controller. Two 980nm/1550nm wavelength division multiplexers(WDM) were used to multiplex and demultiplex the pump beam at 976nm and the signal light near 1550nm, respectively. The interference fringes in the region of 1549–1559nm were monitored upon pumping at the launched pump power from 0 to ~56mW of a laser diode(LD; JDS Uniphase, at 976nm) by using the TLS and the power meter(81531A, Agilent), where the scanning interval of the TLS was 0.01nm.

All-optical ON-OFF switching experiment was carried out using the same LPG pair with the Yb³⁺ doped fiber as shown in Fig. 1(b). From the interference fringe pattern formed by the LPG pair with the Yb³⁺ doped fiber, a destructively interfered wavelength near 1550nm was chosen from the TLS. Then, the pump LD was modulated with a square wave pulse train, in which the lower power was 0mW and the upper power was the launched pump power corresponding to the ‘π’ phase shift. In order to examine the frequency dependence on the full optical switching (‘π’ phase shift), the modulation frequency of the pump signal from the LD driver was varied by using the function generator(33120A, Agilent). The TLS light signal after passing through the LPG pair including the Yb³⁺ doped fiber was detected at the 25GHz visible-infrared(400nm–1650nm) photo-detector(PD; New Focus) and the electrical signal from the PD was monitored in the oscilloscope(TDS220, Tektronix).

 

Fig. 1. Schematic diagram of experimental setup (a) for measuring the pump-induced phase change of the high concentration Yb³⁺ doped aluminosilicate optical fiber using a long-period fiber grating pair and (b) for all-optical ON-OFF switching at λ₁ upon pumping with the square wave pulses at 976nm by varying the frequency.

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3. Results and discussion

The interference fringe pattern at 1549–1559nm and its shift with the launched pump power are shown in Fig. 2(a). The interference fringes were shifted toward the longer wavelength side with the increase of the pump power. Also, the depth of the transmission in the interference fringe pattern, which is related to the optical switching extinction ratio, was distributed from 10dB to 18dB. Figure 2(b) shows the average wavelength shift at three destructively interfered wavelengths near 1550nm with the launched pump power, where the error bars denote the maximum deviation of the wavelength shift, and the corresponding phase shift, Δψ(λ _p ), which was obtained by

where Δ$n_{\mathit{\text{core}}}^{\mathit{\text{NLO}}}$ (z) is the effective refractive index change of the Yb³⁺ doped fiber core at z, λ _p is the wavelength of the fringe, Δλ is the wavelength shift of λ _p and s is the fringe spacing [4]. The phase change increased almost linearly with the pump power. The ‘π’ phase change corresponding to the full switching and the ‘1.53π’ phase change were obtained at the launched pump powers of ~35mW and ~56mW, respectively.

 

Fig. 2. (a) Transmission spectra at 1549–1559nm of the 25.5cm long Yb³⁺ doped optical fiber with the long-period fiber gratings pair upon pumping with the LD at 976nm. (b) Average wavelength shift at three destructively interfered wavelengths near 1550nm in (a) and its calculated phase shift with the launched pump power, where the error bars denote the maximum deviation of the shift.

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Based on the interference fringe pattern result in Fig. 2(a), the wavelength 1555.72nm was selected for the all-optical switching operation because the extinction ratio was largest at that wavelength with 18dB. Figs. 3(a)–(e) show the optical switching results of the 1555.72nm CW light in the LPG pair with the Yb³⁺ doped fiber upon modulating the pump beam with the 1Hz, 50Hz, 100Hz, 200Hz and 250Hz square wave pulse train, respectively. The blue, red and black lines represent the TLS signals, the pump signals and the function generator signals, respectively. The pump signals well followed the function generator signals. However, as the modulation frequency increased, the TLS signals were gradually spread and the OFF states at 250Hz were not clearly seen. So, the maximum frequency of the all-optical ON-OFF switching would be ~200Hz. The falling time at 100Hz and 250Hz was approximately 1ms, which was similar to the ²F_5/2 energy level lifetime(~750µs) of the Yb³⁺ ions in the silica-based glass [2]. Since the nonlinear optical properties of the Yb³⁺ doped fibers, which induce the phase change required for the optical switching, are attributed to the radiative electronic transitions in the Yb³⁺ ions by the pumping [2], the spreading of the TLS signals with the increase of the modulation frequency must be related to the radiative electronic transitions at the ²F_5/2 energy level of the Yb³⁺ ions. Also, note that the modulation bandwidth, Δf, in the intensity modulation of a LED light is related to the total carrier lifetime, τ, as follows [10].

If the carrier lifetime in the LED is regarded as the lifetime of the pumped energy level in the Yb³⁺ ions, the modulation bandwidth in the LPG pair including the Yb³⁺ doped fiber can be estimated to be ~212Hz, which is similar to the results shown in Fig. 3. Therefore, the all-optical switching performance shown in Fig. 3 is well explained by the radiative electronic transitions at the ²F_5/2 energy level of the Yb³⁺ ions in the Yb³⁺ doped fiber.

On the other hand, Fig. 3(a) obtained from the 1Hz modulation showed a slightly additional increase and decrease in the TLS signal amplitude at the ON and OFF states, respectively, without spreading the pulses. The additional response is expected to be due to the thermal effect in the Yb³⁺ doped fiber since the thermal response time in Yb³⁺ doped alumino-germanosilicate fibers was reported to be ~150ms [11]. It is known that the doped fibers have a thermal effect due to the non-radiative electronic transitions in the dopant [3]. The non-radiative transitions will be dissipated into a heat, which increases the temperature of the fiber. However, it was difficult to determine the thermal response time in our experimental results, which would be due to the small contribution of the thermal effect to the phase change in our method because of the one-fiber operation in principle [4]. Basically, the phase change in the LPG pair method is obtained by the change of the optical path difference between the core-guided light and the cladding-guided light of the fiber within the two same LPGs. If the temperature change of the core and the cladding in a single fiber between the LPG pair is the same, no phase change is expected. Since the temperature rise due to the pumping in the core and the cladding of doped fibers is about the same despite the different diameter and glass composition of the core and the cladding [3], the thermal phase change in the LPG pair with the doped fiber would be negligibly small.

 

Fig. 3. All-optical ON-OFF switching results of the 1555.72nm CW light upon pumping at 976nm with the square wave pulses of (a) 1Hz, (b) 50Hz, (c) 100Hz, (d) 200Hz and (e) 250Hz using the function generator (Blue solid lines for the TLS signals, red solid lines for the pump signals and black solid lines for the function generator signals), where the blue dashed lines denote a zero transmission of the TLS signal.

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Based on the thermal response time of ~150ms in the Yb³⁺ doped fiber, the thermal phase change in our experiments was estimated. The maximum modulation amplitude at 50Hz and 100Hz, where the thermal effect was averagely cancelled out, was ~94% compared to that at 1Hz, where the extinction ratio is expected to be ~18dB as in the case of the CW pumping. This means that the extinction ratio at 50Hz and 100Hz would be ~17.7dB. Also, the thermally induced phase change would be ~0.19rad in the ‘π’ phase change for the full optical switching. Note that the thermal effect of doped fibers on the phase change is different depending on the fiber device types, such as the fiber MZ interferometer, the TCF interferometer, the TMF interferometer and so on, and the thermally induced phase change in the ‘π’ phase change should be smaller than 0.2rad to keep the thermal phase bias change under -20dB [3]. Therefore, the LPG pair with the doped fiber would have a negligible thermal effect and be a good candidate as an all-optical switching device.

4. Conclusion

The all-optical ON-OFF switching with the CW laser signal at ~1556nm was demonstrated by employing a long-period fiber gratings pair with the 25.5cm long Yb³⁺ doped aluminosilicate glass fiber(0.82at% Yb³⁺ and 3.02at% Al3+) upon pumping with the square wave pulses at 976nm by varying the frequency in the range of 1–250Hz, where the full optical switching with the ~18dB extinction ratio was obtained at the launched pump power of ~35mW. The switching extinction ratio, and the switching response time and pump power can be more improved by controlling the fabrication parameters of the LPG pair and inserting the higher nonlinear optical fiber with a faster response time, respectively. Also, it was found that the thermal effect of the LPG pair with the doped fiber would be small enough to be utilized in all-optical switching. Therefore, our proposed configuration by employing the LPG pair with the high nonlinear optical fiber will be useful as an all-optical switching device.