MMI filters configuration for dual-wavelength generation in a ring cavity erbium-doped fibre laser
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Dual wavelength laser generation has been of constant interest due to their applications in optical communications and teraheartz generation. A novel configuration for Dual-wavelength laser generation based on the use of a couple of multimode interference (MMI) filters is demonstrated in a ring cavity Erbium-doped fibre laser.
The MMI filters consist of a segment of no-core fibre spliced between two SMF-28 single mode fibre segments. The MMI filters configured as a Mach-Zehnder interferometer, are used as transmission spectral filters for simultaneous generation of two laser wavelengths. An optical attenuator is used to adjust the intra-cavity losses for dual-wavelength laser generation.
Laser emission at 1540.4 and 1554 nm for a wavelength separation of ~13.6 nm is obtained. The laser wavelengths output power stability variations with the applied pump power is also experimentally discussed.
The use of MMI filters in the proposed dual-wavelength laser filter is experimentally demonstrated as a reliable device for dual-wavelength generation in fibre lasers.
KeywordsFibre lasers Multimode interference filter Dual-wavelength laser Erbium-doped fibre
Spectral filters in optical fibre based on single-mode-multimode-single-mode (SMS) fibre structure have been of significant interest as reliable optical devices because of their many advantages such as compatibility for all-fibre integration, low cost, low insertion loss and ease to fabricate. The operation principle of the SMS fibre structure is based on a self-imaging phenomenon described for slab waveguides by Soldano and Penings . The filtering aspects of the SMS fibre structure was discussed by Mohammed et al. . The SMS filter exhibits a narrow transmission spectral width with a leading wavelength peak. Moreover, its transmission wavelength peak can be tuned by external parameters such as temperature, strain and liquid refractive index [3, 4]. This features make the SMS fibre structure attractive for fibre-optical sensing [5, 6, 7] and tuneable fibre laser applications [8, 9, 10]. The performance of the SMS structure as a band-pass filter [8, 9] make it a reliable device for wavelength selection and tuning of ring cavity fibre lasers. Furthermore, when the MMI filter is properly designed, the transmission peak wavelength can be easily selected in terms of the multimodal fibre (MMF) refractive index, core diameter and length [2, 3].
As the dual-wavelength laser emission has generated a considerable interest in recent years, the obtaining of two simultaneous laser wavelengths by using a single cavity has been attractive in different areas such as optical fibre sensing, optical communications, microwave and terahertz generation. However, EDF is a homogeneous gain medium at room temperature, which leads to a strong mode competition for the generated laser lines. Therefore, several techniques to achieve dual-wavelength laser emission where the use of optical filters as a reliable method for cavity losses adjustment have been reported [11, 12, 13, 14, 15], among which can be mentioned fibre Bragg gratings (FBG), Mach-Zehnder interferometer (MZ) and Sagnac interferometer. Recently, tuneable Erbium- and Ytterbium-doped fibre lasers using MMI filters have been reported [8, 9, 10]. However, the potential of using MMI filters for dual-wavelength laser generation, has been underexploited. To our knowledge, only few researches of dual-wavelength fibre lasers in which a SMS structure is used for dual-wavelength generation have been reported [16, 17, 18]. However, the use of a single SMS structure in these investigations, limits the laser lines wavelength separation and make difficult the wavelength selection and the cavity losses adjustment.
In this paper, we demonstrate stable dual-wavelength laser emission of an EDF laser based on the use of two SMS filters. The filters with SMS structure are disposed in a MZ interferometer configuration for separately laser wavelength selection. The dual-wavelength laser lines separation is ~13.6 nm with laser lines wavelength at 1540.4 and 1554 nm. The wavelength is marginally modified with the increase of the pump power. The reliability of using two MMI filters for separately wavelength selection of stable dual-wavelength laser operation is experimentally demonstrated.
MMI dual-wavelength filter principle and characterization
where p is the self-image number, n NCF is the NCF effective refractive index, D NCF is the diameter of the NCF corresponding to the effective width of the fundamental mode, and L is the NCF length. The home-made MMI filters were constructed to obtain the fourth self-image by using a NCF (in which air acts as cladding) with 125 μm diameter. The NCF lengths of 58.7 and 58 mm, to obtain transmission wavelength peaks of 1537.5 and 1554.3 nm (for MMI filter 1 and MMI filter 2 respectively) were calculated with Eq. (1). Fig. 1b shows the transmission of the constructed MMI filters. To measure the transmission, we used a LED source with emission in a wavelength range from 1465 to 1650 nm, as input signal. The transmission of the MMI filters was estimated by the following method: Initially, three spliced segments of SMF-28 fibre were mounted in a metal plate to avoid instability in the estimation, and the fibre ends were connected to bare fibre adaptors. In one of the adaptors the LED source was connected and the input signal spectrum was recorded with an OSA at the other connector. Then, the central segment of SMF-28 fibre was replaced with one of the MMI filters. Likewise, the output signal of the LED source due to the MMI was measured. The transmission of both separately MMI shown in Fig. 1b was estimated as the measured MMI output signal divided by the measured LED source output signal. As it can be observed, the transmission spectrum of the MMI filter 1 (blue line) exhibits a wavelength peak at ~1538.6 nm and for the MMI filter 2 (red line) a wavelength peak is observed at ~1554.3 nm. The FWHM of both MMI filters is around 11 nm. The transmission losses for the MMI filter 1 and MMI filter 2 are 19 % and ~14 %, respectively.
Results and discussions
Figure 4b shows repeated measurements of the dual-wavelength laser output with a launched pump power of 100 mW. The OA was adjusted to obtain dual wavelength laser operation with equal power wavelengths. Once simultaneous laser emission was reached, the OA was fixed to prevent instability by deformation. A set of ten measurements with a 2 min interval were obtained at room temperature in which thermal dependence of the MMI-DWF is not noticeable. The peak power variation for each generated wavelength was less than 0.3 %. Therefore, dual-wavelength laser emission stability is observed.
The laser generation depends on the EDF amplification spectrum gain and on the filter transmission at the specific wavelength. The laser wavelength which will be generated is defined by the maxima of the product of the filter transmission and the EDF amplification. However, each MMI filter exhibits a narrow wavelength range of ~ 2 nm were the transmission maxima is approximately the same. Therefore, when the pump power is changed, the EDF amplification spectrum undergoes a profile modification that leads to a laser line generation in a slightly shifted wavelength. We attributed the larger shift of the longer wavelength to the fact of λ2 is generated in a wavelength range in which the EDF amplification spectrum exhibits a downward slope of gain which leads to a more pronounced wavelength shift when pump power is varied.
In this paper, stable dual-wavelength laser emission using two MMI filters (configured as a MZ interferometer) was experimentally demonstrated. The novel filter structure was used for individual wavelengths selection and cavity losses adjustment to achieve dual-wavelength laser operation of an EDF ring cavity laser. With a pump power of 100 mW, dual-wavelength laser emission at 1540.4 and 1554 nm with a wavelengths separation of 13.6 nm was achieved. The laser wavelengths output power stability variations with the applied pump power was also experimentally discussed. The wavelength of the generated laser lines slightly shifts toward shorter wavelengths with an increase of the applied pump power. The obtained results demonstrate the reliability of the proposed MMI-DWF for dual-wavelength laser generation in an all-fibre ring cavity fibre laser with potential applications in optical communications and sensing.
Manuel Durán-Sánchez was supported by Cátedras-CONACyT project 2728. R. I. Álvarez-Tamayo wants to thanks CONACyT postdoctoral fellow 160248. B. Ibarra-Escamilla was supported by CONACyT Grants No. 237855 and No. 255284.
RIAT and MDS conceived the work. MDS did the implementation, obtained the experimental results and contributed to results analysis. RIAT did results analysis and contributed to writing. JGAS did the MMI filters design and analysis. JEAL developed and fabricated the no-core fibre and contributed to writing. BIA and EAK coordinated the work, did result analysis and contributed to writing. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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