Broad, Tunable and Stable Single-Frequency Erbium Fiber Compound-Ring Lasers Based on Parallel and Series Structures in L-Band Operation

Abstract

In this demonstration, we present two erbium-doped fiber (EDF) lasers, with series and parallel three sub-ring configurations, respectively, to achieve tunable channel output and stable single longitudinal mode (SLM) operation in the L-band range. Here, the fiber ring cavity contains the L-band EDF as a gain medium. Based on the measured results of the two quad-ring structures of the EDF lasers, tunable output bandwidth for the two lasers can be obtained from 1558.0 to 1618.0 nm simultaneously. All the 3 dB linewidths measured for both fiber lasers are 312.5 Hz over the effective wavelength output range. Furthermore, the related optical signal-to-noise ratio (OSNR), output power, output stabilities of the central wavelength and power, and equal output power range of the two proposed EDF lasers are also examined and discussed.

1. Introduction

With the progress of advanced technology in erbium-doped fiber (EDF)-based lasers, the output characteristics of stable single longitudinal mode (SLM), wide wavelength tunability, high optical signal-to-noise ratio (OSMR) and narrow linewidth will become crucial [1,2,3]. EDF-based lasers can be applied in many useful applications, such as fiber-optic sensors, optical fiber communications, bio-photonic, microwave-photonic and wavelength division multiplexing (WDM) transmission, and spectroscopy [4,5,6,7,8]. The structures of commonly constructed EDF lasers have linear and ring-based configurations [9,10]. Furthermore, ring-based EDF laser architecture is a good choice to attain a narrow linewidth and high power output owing to the long gain medium and fiber cavity [11]. However, mode-hopping and multi-longitudinal mode (MLM) effects are caused by the homogeneous broadening of the EDF and the long fiber cavity length [12]. How to obtain and maintain stable SLM output from EDF lasers is therefore an essential research topic. Hence, some relevant technologies have been studied and demonstrated, such as applying various compound-ring designs [13,14,15], using an optical ultranarrow bandpass filter [16], designing a Rayleigh backscattering (RB) feedback injection loop [17], utilizing a Mach–Zehnder interferometer (MZI) structure-based filter operation [18] and using an unpumped saturable absorber (SA)-based filter effect [19]. Among these proposed methods, using multiple-ring cavity architecture is the simplest and best selection to achieve SLM operation due to the wide and effective free spectral range (FSR_eff)-based mode filter caused by the Vernier effect for dense MLM suppression [14,20,21]. This is why it is necessary to design and use compound-ring cavities in fiber laser architecture to obtain SLM output.

In this paper, we present two EDF quad-ring laser structures with series and parallel sub-rings to reach stable and selectable SLM wavelength outputs. An L-band EDF is placed in the fiber ring cavity as a gain medium for the L-band operation. In the measurement, based on the Vernier effect causing a mode filter effect through the quad-ring, both EDF lasers can achieve SLM output over the same tuning bandwidth of 1558.0 to 1618.0 nm. Therefore, the two proposed EDF lasers can not only achieve SLM operation, but can also obtain a broad wavelength-tuning bandwidth. Moreover, the 3 dB linewidths of the two EDF lasers measured are 312.5 Hz over the achievable tuning bandwidth. To identify and compare the other output characteristics of the two EDF lasers, relevant experiments and more discussions will be presented in Section 2 (series sub-ring) and Section 3 (parallel sub-ring), respectively.

2. Series Sub-Ring-Based EDF Laser

Figure 1 displays the EDF compound-ring laser architecture demonstrated by three sub-rings in series in this section. This fiber laser is constructed by three 2 × 2 50:50 optical couplers (CPR1s), an L-band erbium-doped fiber amplifier (EDFA), a polarization controller (PC), a tunable bandpass filter (TBF) and a 1 × 2 10:90 optical coupler (CPR2). The gain medium of the EDF multi-ring laser applies the L-band EDFA, having 10 dBm saturation output over the achievable bandwidth of 1568 to 1604 nm, to generate the wavelength output. In the experiment, all the fiber used is single-mode fiber and the gain of the EDFA is 30 dB in the obtainable operation bandwidth. The corresponding polarization direction of the fiber ring laser can be properly adjusted by the PC to reach optimal and stable output power. Furthermore, a TBF with an adjustable range of 1510 to 1630 nm is inserted into the main fiber ring cavity to filter the corresponding wavelength for wavelength output. The 90% port of the CPR2 is used to connect the EDFA, three series sub-rings, PC and TBF to generate the main fiber ring (R_main), as shown in Figure 1. To test and measure the output wavelength spectrum, an optical spectrum analyzer is operated to connect the 10% port of the CPR2.

As seen in Figure 1, three CPR1s can cause three sub-rings in series, represented by R₁, R₂ and R_3, respectively. Here, four fiber rings (R_main, R₁, R₂ and R₃) are generated to produce a quad-ring EDF laser configuration. Moreover, each fiber ring has its corresponding free spectral range (FSR) due to the various fiber lengths. Therefore, the R_main, R₁, R₂ and R₃ will have a corresponding FSR_main, FSR₁, FSR₂ and FSR₃, respectively. When the four FSRs meet the least common multiple according to the Vernier operation [13,14], the maximum effective FSR range can be obtained, thereby producing a mode-filtering effect and mitigating dense longitudinal mode oscillations. In this demonstration, the fiber lengths of R_main, R₁, R₂ and R₃ are 22, 1.23, 2 and 3 m, respectively. So, their corresponding FSRs (FSR_main, FSR₁, FSR₂ and FSR₃) of 9.28, 166.15, 102.18 and 68.12 MHz can be obtained via the expression of FSR = c/(n · L), where L, n and c represent the fiber ring length, average index of the fiber (1.468) and speed of light in a vacuum (3 × 10⁸ m/s). Based on the Vernier effect, the most effective FSR of 10.732 THz occurs when filtering multiple spaced longitudinal modes. The effectual FSR diagram of the SLM mode selection consistent with the Vernier effect is illustrated in Figure 2. Moreover, the 3 dB bandwidth of the TBF is 50 GHz (0.4 nm). Thus, the densely spaced longitudinal modes can be suppressed fully due to the effective FSR-induced mode filter effect (>50 GHz).

First, to confirm the effective tunability range of the designed EDF quad-ring laser with series sub-rings, the different passband of the TBF can be controlled from short wavelength to long wavelength for generating various wavelength outputs. So, the output lased wavelength of this fiber laser starts at a wavelength of 1558.0 nm and ends at 1618.0 nm, as shown in Figure 3. Moreover, Figure 3 also displays the measured amplified spontaneous emission (ASE) profile of the EDFA as a dashed line. The effective ASE bandwidth is around the L-band scale to match the wavelength-lasing bandwidth of the EDF laser. Seven representative output wavelengths of the proposed EDF laser from 1558.0 to 1618.0 nm are plotted in Figure 3. The measured results show that the quad-ring-induced mode filter effect can spread the wavelength-tuning scale on both sides. Through the provided EDF compound-ring laser structure, the available wavelength-lasing bandwidth is up to 60 nm. Based on the quad-ring design, all the observed ASE background noise at seven selected output wavelengths is completely suppressed. As seen in Figure 3, all the measured optical noise of each output wavelength is less than −71 dBm. This means that in addition to broadening the wavelength tunable range, the quad-ring-structure-induced mode filter effect also has a high degree of background noise suppression appearance.

In addition, the corresponding output power (P) and optical signal-to-noise ratio (OSNR) of the EDF quad-ring laser are also measured within the available wavelength bandwidth of 1558.0 to 1618.0 nm, as exhibited in Figure 4. A tuning step of 5 nm is selected to measure the related output characteristics of the lasing wavelength for observation. We can also adjust the passband of the TBF to generate continuous wave output. The obtained output power and OSNR ranges for each lasing wavelength are between −16.5 and −0.49 dBm and between 71.16 and 110.74 dB, respectively. In general, an additional optical filter can be added inside the laser cavity to improve the OSNR. The closer the measurements are taken to both sides of the usable wavelength range, the lower the output power measured. This is because the gains on both sides are smaller. However, the corresponding OSNR will be relatively high, as seen in Figure 3. An output power difference of 0.87 dB can be spread over the wavelength range of 1573.0 to 1598.0 nm for equal output power. However, the corresponding OSNR is distributed between 71.79 and 78.25 dB in the same flattened wavelength scope.

To confirm whether the output wavelength of this fiber laser is an SLM operation, we can use a delayed self-homodyne setup to measure and test this. This setup is constructed from a two-arm MZI used for SLM observations. One arm has nearly 75 km of fiber delayed line. Moreover, a PIN-based photodiode is applied to receive the optical beat signals via the MZI and convert them into electrical signals for measurement by an electrical spectrum analyzer. Therefore, seven output wavelengths from 1558.0 to 1618.0 nm were also selected with 10 nm channel spacing to verify the SLM output. Through the self-homodyne setup, the observed electrical output spectra within 1000 MHz frequency range for the seven selected wavelengths are identified and displayed in Figure 5, respectively. All the measured electrical spectra at the seven wavelengths do not contain other longitudinal mode oscillations, as seen in Figure 5. This measurement result also directly proves that the quad-ring fiber laser has the characteristics of SLM output. Then, we also can apply MZI-based self-heterodyne detection to execute the laser linewidth. Here, a shift frequency of 55 MHz is added onto an acousto-optic modulator in the optical setup for a beating signal. First, an output wavelength of 1588.0 nm is applied through the test setup for linewidth measurement. So, Figure 6 shows the measured electrical linewidth at a wavelength of 1588.0 nm with a center frequency of 55 MHz and an observation bandwidth of 0.04 MHz as a circle symbol. And we can confirm the actual laser linewidth by fitting the measured results to a Lorentzian curve, as illustrated by the solid line in Figure 6. A 3 dB Lorentzian linewidth of 312.5 Hz is achieved at the 1588.0 nm wavelength.

Furthermore, to realize all the 3 dB Lorentzian linewidths of each wavelength produced by the presented quad-ring fiber laser, the wavelength-tuning scope starts at a 1518.0 nm wavelength and ends at 1618.0 nm through the same experiment as mentioned above. And the wavelength-tuning step is ~5 nm for the linewidth observation. So, Figure 7 presents the corresponding 3 dB Lorentzian linewidth in an available wavelength bandwidth of 1518.0 to 1618.0 nm. The entire linewidths are obtained at 312.5 Hz over the same tuning range. As a result, the proposed quad-ring structure not only reaches a wider wavelength-selectable bandwidth, but also obtains a narrower linewidth of sub-kHz.

The output stability of the proposed quad-ring laser with three series sub-rings is also an important subject. Here, we also pick a wavelength of 1588.0 nm for the stability measurement. The changes to the central wavelength λ and output power P at 1588.0 nm wavelength are observed through a measurement time of 40 min. Thus, the detected λ_c and P change range is from 1587.992 to 1588.016 nm and −3 to −2.76 dBm, respectively, as seen in Figure 8a. This also indicates that the maximum oscillations of central wavelength and output power are less than 0.024 nm and 0.24 dB, respectively. Finally, the largest oscillations of wavelength and power for each lasing wavelength over the tuning bandwidth of 1558.0 to 1618.0 nm are also measured through the same period of 40 min. In this experiment, the related central wavelength and power output of each lasing wavelength are tested over the same tunability range to observe the maximum corresponding fluctuations Δλ and ΔP after a measurement time of 40 min, as displayed in Figure 8b. The related oscillations of the central wavelength and output power are measured between 0.016 and 0.048 nm and 0.04 and 1.19 dB in the available wavelength bandwidth, respectively, as seen in Figure 8b. The maximum values of Δλ and ΔP both appear at the wavelength of 1558.0 nm.

3. Parallel Sub-Ring-Based EDF Laser

Figure 9 presents the EDF compound-ring laser system demonstrated by three cascaded sub-rings in this section. This fiber ring laser also consisted of three 2 × 2 50:50 CPR1s, an L-band EDFA, a PC, a TBF and a 1 × 2 10:90 CPR2. Moreover, the 90% port of the CPR2 is connected to the EDFA, parallel three sub-rings, PC and TBF to create a main fiber ring (R_main), as seen in Figure 9. The three sub-rings in parallel are generated by the three CPR2s. In the experiment, the fiber lengths of the R_main, R₁, R₂ and R₃ are the same as those stated in the previous section. Thus, based on the Vernier effect, the most effective FSR of 10.732 THz is also achieved to suppress the multiple spaced longitudinal modes for obtaining the SLM operation.

To compare the EDF ring laser with the series sub-ring structure, we will first also confirm whether there is a difference in the adjustable wavelength range. In this measurement, the generated wavelength of the proposed quad-ring fiber laser starts at the 1558.0 nm wavelength and concludes at 1618.0 nm, as displayed in Figure 10. So, the same wavelength-tuning scope (bandwidth = 60 nm) of the quad-ring EDF laser is reached. The ASE background noise of each lasing wavelength can be mitigated completely and the optical noise is suppressed to below −71 dBm, as shown in Figure 10. Therefore, the optical output characteristics obtained by the proposed EDF laser are the same as those of the series sub-ring structure.

The OSNRs and output powers of the presented EDF laser with parallel sub-ring structure are exhibited over the wavelength scale of 1558.0 to 1618.0 nm, as shown in Figure 11. The measured output power P and OSNR spans for each lasing wavelength are between −16.7 and −2.7 dBm and between 70.49 and 114.81 dB, respectively. The closer the measurements are taken to both sides of the usable wavelength range, the lower the output power measured. This is because the effective gains on both sides are smaller. However, its corresponding OSNR will be relatively high, as seen in Figure 3. The highest OSNR of 114.81 dB (P = −4.9 dBm) occurs at the 1608.0 nm wavelength. An output power difference of <1 dB can be spread over the wavelength range of 1573.0 to 1603.0 nm for equal output power (P = −3.8 to −4.8 dBm). However, the corresponding OSNR is distributed between 70.49 to 110.89 dB in the achieved flattened wavelength scope.

Then, we also can apply the delayed self-homodyne to confirm the SLM operation of each output wavelength. In this measurement, seven output wavelengths from 1558.0 to 1618.0 nm are also chosen with 10 nm channel spacing to prove the SLM output. Thus, the observed electrical output spectra within a 1000 MHz frequency range for the seven selected wavelengths are displayed in Figure 12. The whole electrical spectra at seven wavelengths do not include other longitudinal mode oscillations, as seen in Figure 12. This result shows that the quad-ring fiber laser also has the characteristics of SLM function. Next, we also use the same self-heterodyne detection to measure the laser linewidth. Here, a shift frequency of 55 MHz is added to the setup for beating signal. First, an output wavelength of 1588.0 nm is used for linewidth demonstration. Thus, Figure 13 presents the detected electrical linewidth at a wavelength of 1588.0 nm with a center frequency of 55 MHz and an observation bandwidth of 0.04 MHz as a circle symbol. To obtain the actual linewidth, a Lorentzian curve is utilized to fit the measured result, as demonstrated by the solid line in Figure 13. The 3 dB Lorentzian linewidth of 312.5 Hz is obtained at the 1588.0 nm wavelength. Furthermore, we also execute a linewidth measurement based on the Lorentzian profile over the obtainable wavelength bandwidth of 1558.0 to 1618.0 nm. And the 3 dB linewidth at each output wavelength is measured at 312.5 Hz.

Finally, the related central wavelength and power output of each lasing wavelength are measured over the same wavelength-tuning bandwidth to observe the maximum corresponding fluctuations in Δλ and ΔP after a measurement time of 40 min, as displayed in Figure 14. The related oscillations of the central wavelength and output power are measured between 0.016 and 0.024 nm and 0.21 and 0.42 dB in the available bandwidth of 1558.0 to 1618.0 nm, respectively, as seen in Figure 14. This result shows that within the effective wavelength adjustable range, the maximum output power and central wavelength drift will be less than 0.42 dB and 0.024 nm in the whole tuning range.

4. Discussion

According to the measurement results in Section 2 and Section 3, the two proposed EDF ring lasers with series and parallel sub-ring configurations have the same wavelength-tuning bandwidth in the range of 1558.0 to 1618.0 nm. The two quad-ring-based EDF lasers can create a mode filter effect to mitigate the multiple longitudinal modes and also extend the effective gain range for broadband wavelength selection. The mode filter function also enables the OSNR obtained to be greater than 70.49 dB. Furthermore, both EDF lasers demonstrated are also capable of SLM operation and have the same 3 dB linewidth of 312.5 Hz over the available wavelength range. In this discussion, a comparison of the output characteristics of fiber laser 1 (series sub-ring) and laser 2 (parallel sub-ring) is presented in

Table 1. Comparison of the output characteristics of fiber laser 1 (series sub-ring) and laser 2 (parallel sub-ring).

Laser Configuration	Series Sub-Ring (1)	Parallel Sub-Ring (2)
Effective Tuning Range (nm)	1558.0 to 1618.0	1558.0 to 1618.0
3 dB Linewidth (Hz)	312.5	312.5
Output Power (dBm)	−16.5 to −0.49	−16.7 to −2.7
OSNR (dB)	71.16 to 110.74	70.49 to 114.81
Wavelength Fluctuation (nm)	0.016 to 0.048	0.016 to 0.024
Power Oscillation (dB)	0.04 to 1.19	0.21 to 0.42
Equal Output Power Range	1573.0 to 1598.0 nm (ΔP ≤ 0.87 dB)	1573.0 to 1603.0 nm (ΔP ≤ 1.03 dB)

. Due to the effective gain spreading effect, the output power of the two demonstrated EDF lasers is relatively small, as seen in Table 1. The obtained output power of the proposed fiber laser 1 will be slightly greater than that of fiber laser 2. The central wavelength and output power variation range of fiber laser 2 is relatively uniform and small. However, fiber laser 2 has a better OSNR, wavelength fluctuation and output power oscillation than laser 1. Moreover, fiber lasers 1 and 2 had a similar equal output power range based on a quad-ring design with series and parallel sub-ring architecture. Compared with past works [21,22,23,24], the series and parallel three sub-ring fiber laser architecture proposed is simple and can easily achieve a wavelength linewidth of 312.5 Hz and high OSNR of 70.49 to 114.81 dB. The results presented by the EDF laser are superior to those mentioned in prior studies [21,22,23,24].

5. Conclusions

In summary, two EDF laser structures based on quad-ring designs with series and parallel sub-ring were proposed, respectively. An L-band EDFA was used as the gain medium inside the proposed two fiber ring cavities. The mode-filtering effect caused by the quad-ring structure could suppress the dense longitudinal modes of the SLM operation and spread the operable gain range. In the demonstration, the two quad-ring-based lasers also completed a 60 nm wide wavelength-selectable bandwidth from 1558.0 to 1618.0 nm in parts of the C- and L-bands. All the measured 3 dB linewidths for both EDF ring lasers were 312.5 Hz over the achievable tuning bandwidth. This also proved that the fiber laser with a quad-ring design not only obtained a narrower SLM linewidth output, but also extended the wavelength switchable range. Moreover, the measured OSNRs of the two EDF lasers were between 70.49 and 114.81 dB. However, due to the gain extension effect of the fiber laser caused by the mode filter effect, the output power obtained under the quad-ring-based EDF laser would be slightly lower, between −16.5 and −2.7 dBm. The two sub-ring structures also attained a flat power output range from 1573.0 to 1598.0 nm (25 nm bandwidth) with a power variation range of <1 dB. The observed greatest oscillations of Δλ and ΔP remained in the range of 0.016 to 0.048 nm and 0.04 to 1.19 dB, respectively, in the proposed two EDF lasers. Here, the maximum power fluctuation ΔP of 1.19 dB was only caused at the wavelength of 1558.0 nm in the series sub-ring-based fiber laser. As a result, of the two proposed EDF lasers, there was little difference in the wavelength-dependent output characteristics obtained.